Cleanup: Consistent indent style for asset CMakeLists file

Was mixing 4 and 2 space indent in a single file.

Cleanup: Pass asset handle to asset iterator, rather than wrapped file

This iterator was introduced before `AssetHandle` existed, so it was
dealing with the file data directly. Now we want as little code as
possible to deal with the file data, all access should happen via the
`AssetHandle`.

Assets/UI: Sanity check argument for UILayout.template_asset_view()

Was already doing some sanity checks, but wasn't checking if the passed
property actually is a collection property, which is important.

Cleanup: Remove unnecessary code for asset view UI template

From what I can tell there is no reason anymore to do this. The design
has changed since this was added.

Assets/UI: Resolve major asset view UI template limitation

Before this, all asset view templates showing the same asset library
would show the same assets, even if they should show different ID types.
That was a major limitation since the design did forsee that this
template can be put anywhere in the UI to display various sub-sets of
assets.

Initially I did the ID type filtering close to the asset-list reading,
because I wanted to optimize reading so that we would only actually read
asset information from disk of the ID type to be shown. But this will be
quite complex and I'm not sure if I'll get to work on this anytime soon.
So this commit moves the filtering to the template display level solving
this limitation.

Note: This also adds the code to filter by tags, together with the ID
type. But it's not actually used anywhere yet.

Assets: Improve error message when "Clear Asset" fails

When using "Clear Asset" from the Asset Browser but with an asset
selected that is not stored in the current file, we can show a more
informative error message.

Fix menu poll function being ignored for UILayout.menu

Using `UILayout.menu()` [1] or `UILayout.menu_contents() [2], the menu
would just always be added, the `poll()` check not being executed. As
API user I would expect the `poll()` to deterimine visiblity of the
menu.

[1] https://docs.blender.org/api/current/bpy.types.UILayout.html#bpy.types.UILayout.menu
[2] https://docs.blender.org/api/current/bpy.types.UILayout.html#bpy.types.UILayout.menu_contents

Differential Revision: https://developer.blender.org/D12053

Reviewed by: Campbell Barton

Cleanup: Move reorganize asset files

I'm trying to move away from general files with lots of things in them,
and instead have many small & focused files. I find that easier to
work with since everything has clear responsibilities, even if there is
some minor overhead in managing all these files.
I also try to differentiate more clearly between public and internal
files. So source files and internal headers are in a `intern/`
sub-directory, public functions are in a number of headers one level
higher.
For convenience and to make this compatible with our existing general
headers in `editors/include`, I made the `ED_asset.h` there include all
these public headers.
This is of course a bit of an experiment, let's see how it works in
practice.

Also corrected the name of `ED_asset_can_make_single_from_context()`.

Cleanup: Add missing doxygen file/group comment in new asset file

Assets: Rename workspace active asset library DNA variable

This new variable was introduced with 7898089de3f2. We don't usually use
an `active` prefix variable. Plus, this makes the name match the one of
the Asset Browser active library variable, so we can use the
`rna_def_asset_library_reference_common()` helper for both.

This will cause Asset Views to open with the default "Current File"
Asset Library. We could avoid that, but it's a minor issue really.