Cleanup: Use const for UI icon getter function

Asset Browser: Python mixin utility for category-specific panels

Using this mixin for a panel definition, it's possible to set in which
categories the panel should appear. This is used by the Pose Library
add-on.

UI: New button/widget type for Asset Browser like preview tiles

This button type shows a preview image above centered text, similar to
the File Browser files in Thumbnail Display Mode or the default Asset
Browser display. In fact we may want to port these over to use the new
button type at some point.

Will be used by the asset view UI template that will be added in a
following commit. That is basically a mini version of the Asset Browser
that can be displayed elsewhere in the UI.

Fix T88281: Pose Library 'flip pose' sometimes flips wrong

Correct cases where the X-axis of the bone (in pose space) aligns with
the pose-space Y or Z-axis. In these cases the decomposition of the
matrix fails, and a negative scale of the X-axis turns into a 180�
rotation around the Y-axis. An extra -1 scale to the X and Z axes of the
resulting matrix seems to fix things.

UI/Assets: Initial Asset View UI template

The asset view UI template is a mini-version of the Asset Browser that
can be placed in regular layouts, regions or popups. At this point it's
made specifically for placement in vertical layouts, it can be made more
flexible in the future.
Generally the way this is implemented will likely change a lot still as
the asset system evolves.

The Pose Library add-on will use the asset view to display pose
libraries in the 3D View sidebar.

References:
* https://developer.blender.org/T86139
* https://code.blender.org/2021/06/asset-browser-project-update/#what-are-we-building
* https://code.blender.org/2021/05/pose-library-v2-0/#use-from-3d-viewport

Notes:
* Important limitation: Due to the early & WIP implementation of the
asset list, all asset views showing the same library will show the
same assets. That is despite the ID type filter option the template
provides. The first asset view created will determine what's visible.
Of course this should be made to work eventually.
* The template supports passing an activate and a drag operator name.
The former is called when an asset is clicked on (e.g. to apply the
asset) the latter when dragging (e.g. to .blend a pose asset). If no
drag operator is set, regular asset drag & drop will be executed.
* The template returns the properties for both operators (see example
below).
* The argument list for using the template is quite long, but we can't
avoid that currently. The UI list design requires that we pass a
number of RNA or custom properties to work with, that for the Pose
Libraries should be registered at the Pose Library add-on level, not
in core Blender.
* Idea is that Python scripts or add-ons that want to use the asset view
can register custom properties, to hold data like the list of assets,
and the active asset index. Maybe that will change in future and we
can manage these internally.

As an example, the pose library add-on uses it like this:
```
activate_op_props, drag_op_props = layout.template_asset_view(
"pose_assets",
workspace,
"active_asset_library",
wm,
"pose_assets",
workspace,
"active_pose_asset_index",
filter_id_types={"filter_action"},
activate_operator="poselib.apply_pose_asset",
drag_operator="poselib.blend_pose_asset",
)
drag_op_props.release_confirm = True
drag_op_props.flipped = wm.poselib_flipped
activate_op_props.flipped = wm.poselib_flipped
```

UI: Support defining UI lists in C

So far all UI lists had to be defined in Python, this makes it possible
to define them in C as well. Note that there is a whole bunch of special
handling for the Python API that isn't there for C. I think most
importantly custom properties support, which currently can't be added
for C defined UI lists.

The upcoming asset view UI template will use this, which needs to be
defined in C.

Adds a new file `interface_template_list.cc`, which at this point is
mostly a dummy to have a place for the `ED_uilisttypes_ui()` definition.
I plan a separate cleanup to move the UI-list template to that file.

UI: Auto-scroll to keep active text buttons in view

If a text button is activated that is not in view (i.e. scrolled away),
the scrolling will now be adjusted to have it in view (with some
small additional margin). While entering text, the view may also be
updated should the button move out of view, for whatever reason. For the
most part, this feature shouldn't be needed and won't kick in, except
when a clicked on text button is partially out of view or very close to
the region edge. It's however quite important for the previously
committed feature, that is, pressing Ctrl+F to start searching in a UI
list. The end of the list where the scroll button appears may not be in
view. Plus while filtering the number of visible items changes so the
scrolling has to be updated to keep the search button visible.

Note that I disabled the auto-scrolling for when the text button spawned
an additional popup, like for search-box buttons. That is because
current code assumes the button to have a fixed position while the popup
is open. There is no code to update the popup position together with the
button/scrolling.

I also think that the logic added here could be used in more places,
e.g. for the "ensure file in view" logic the File Browser does.

UI: Support pressing Ctrl+F over UI lists to search

Adds an operator invoked by default with Ctrl+F that while hovering a UI
list, opens the search field of the list and enables text input for it.
With this commit the search button may actually be out of view after
Ctrl+F still. The following commit adds auto-scroll to solve that.

A downside is that in the Properties, there also is Ctrl+F to start
the editor-wide search. That's not unusual in Blender though (e.g.
scolling with the mouse over a UI list also scrolls the list, not the
region).

UI: Support UI list tooltips, defined via Python scripts

Makes it possible to create tooltips for UI list rows, which can be
filled in .py scripts, similar to how they can extend other menus. This
is used by the (to be committed) Pose Library add-on to display pose
operations (selecting bones of a pose, blending a pose, etc).

It's important that the Python scripts check if the UI list is the
correct one by checking the list ID.
For this to work, a new `bpy.context.ui_list` can be checked. For
example, the Pose Library add-on does the following check:
```
def is_pose_asset_view() -> bool:
# Important: Must check context first, or the menu is added for every kind of list.
list = getattr(context, "ui_list", None)
if not list or list.bl_idname != "UI_UL_asset_view" or list.list_id != "pose_assets":
return False
if not context.asset_handle:
return False
return True
```

UI: Support left-right arrow key walk navigation in UI lists

Add improved arrow key walk navigation in grid layout UI List templates.
Pressing up or down walks the active item to the adjacent row in that
direction, while left and right walk through the items along the columns
wrapping at the rows.

Note from Julian:
In combination with the following commit, this has the important
limitation that the list's custom activate operator won't be called when
"walking over" an item that is scrolled out of the list. That is because
we don't actually create any buttons for those that could be used for
the handling logic. For our purposes of the pose libraries that should
be fine since the asset view list is always made big enough to display
all items. Solving this might be difficult, we don't properly support
nesting boxes with proper scrolling in regular layouts. It's all just
hacked a bit for UI-lists to work. Overlaps quite a bit with T86149.

Differential Revision: https://developer.blender.org/D11063

UI: Internal support for custom UI list item drag & activate operators

For pose libraries, we need to be able to apply a pose whenever
activating (clicking) an item in the Pose Library asset view and blend
it by dragging (press & move). And since we want to allow Python scripts
to define what happens at least when activating an asset (so they can
define for example a custom "Apply" operator for preset assets), it
makes sense to just let them pass an operator name to the asset view
template. The template will be introduced in a following commit.

UI: New UI list layout type for big preview tiles

This new layout type is meant for the upcoming asset view UI template.
With it it is possible to show big asset previews with their names in a
responsive grid layout.

Notes:
* The layout is only available for C defined UI lists. We could expose
it to Python, but I think there are still some scrolling issues to be
fixed first. (The asset view template doesn't use scrolling for the UI
list.)
* I'd consider this a more usable version of the existing `GRID` layout
type. We may remove that in favor of the new one in future.

UI: UI list refactor & preparations for asset view template

This is more of a first-pass refactor for the UI list template. More
improvements could be done, but that's better done separately. Main
purpose of this is to make the UI list code more manageable and ready
for the asset view template.

No functional changes for users.

* Split the huge template function into more manageable functions, with
clear names and a few structs with high coherency.
* Move runtime data management to the template code, with a free
callback called from BKE. This is UI data and should be managed at
that level.
* Replace boolean arguments with bit-flags (easily extendable and more
readable from the caller).
* Allow passing custom-data to the UI list for callbacks to access.
* Make list grip button for resizing optional.
* Put logic for generating the internal UI list identifier (stored in
.blends) into function. This is a quite important bit and a later
commit adds a related function. Good to have a clear API for this.
* Improve naming, comments, etc.

As part of further cleanups I'd like to move this to an own file.

Assets: temporarily apply pose when generating preview image

When generating a preview image for a pose, temporarily apply it to the
armature. Contrary to the usual pose application, this ignores the
selected bones and always applies the entire pose.

Assets: Expose active asset library in context

For the Asset Browser, this returns the active asset library of the
Asset Browser, otherwise it returns the one active in the workspace.

This gives simple access to the active asset library from UI code and
Python scripts. For example the upcoming Pose Library add-on uses this,
as well as the upcoming asset view template.

Animation: add function to blend Action into pose

Add function `BKE_pose_apply_action_blend()`, which blends a given
Action into current pose. The Action is evaluated at a specified frame,
and the result is applied to the armature's pose.

A blend factor can be given to blend between the current pose and the
one in the Action. Quaternions are interpolated with SLERP; it is
assumed that their FCurves are consecutively stored in the Action.

This function will be used in the upcoming new Pose Library.

Assets: Add an active asset library per workspace, for the UI to use

This per-workspace active asset library will be used by the asset views
later. Note that Asset Browsers have their own active asset library,
overriding the one from the workspace.

As part of this the `FileSelectAssetLibraryUID` type gets replaced by
`AssetLibraryReference` which is on the asset level now, not the
File/Asset Browser level. But some more work is needed to complete that,
which is better done in a separate commit.
This also moves the asset library from/to enum-value logic from RNA to
the editor asset level, which will later be used by the asset view.

Animation: apply pose to all or selected bones of armature

New function `BKE_pose_apply_action_all_bones()`, which will be
necessary for the upcoming pose library v2.0.

This renames the function `BKE_pose_apply_action` to
`BKE_pose_apply_action_selected_bones`, to reflect that it only works on
selected bones, to contrast it to the new function.

Animation: new pose library based on Asset Browser

Introduce new pose library, based on the Asset Browser. Contrary to the
old pose library (in `editors/armature/pose_lib.c`), which stored an
entire library of poses in an `Action`, in the new library each pose is
its own `Action` datablock. This is done for compatibility with the
asset browser, and also to make it easier to attach preview images,
share datablocks, etc. Furthermore, it opens the door to having
animation snippets in the pose library as well.

This commit contains the C code for the pose library; in order to fully
use it, an addon is required as well (which will be committed shortly).

Assets: Open Blend File operator

Add operator 'Open Blend File' to the Asset Browser. This operator:
- starts a new Blender process,
- opens the blend file containing the asset,
- monitors the new Blender process, and when it stops,
- reloads the assets to show any changes made.