Merge branch 'master' into asset-browser

Fix leftover "focused_id", causing "Make Asset" on material slots to fail

Fix "Make Asset" failing for collections in the Outliner

All ID types but collections have a `TreeStoreElem.type` of 0, but collections...
AFAIK to avoid compatilibity breaking changes back in 2.8.

Fix variable shadowing warning

Sync with review branches, adds "Remove Asset-Data" support

Mostly internal changes. But also adds removing of asset data via the "Remove
Asset-Data" operator in the Outliner and button context menus, as well as
through deleting asset files in the Asset Browser.

Merge branch 'master' into asset-browser

Merge branch 'master' into asset-browser

Merge branch 'temp-review-asset-browser-ui' into asset-browser

Merge branch 'master' into asset-browser

Merge branch 'temp-review-asset-filelists' into temp-review-asset-browser-ui

Merge branch 'temp-review-asset-repositories' into temp-review-asset-filelists

Merge branch 'temp-review-asset-drag-drop' into temp-review-asset-custom-repositories

Merge branch 'temp-review-asset-previews' into temp-review-asset-drag-drop

Merge branch 'temp-review-asset-create-ui' into temp-review-asset-previews

Merge branch 'temp-review-asset-metadata' into asset-metadata-creation

Merge branch 'master' into tmp-review-asset-metadata

Sync with review branches

Some minor changes (mostly cleanups) done while splitting up the branch for review.

Asset Browser UI

This introduces the UI part of the Asset Browser.

Additions:
* Asset Browser as sub-editor of the File Browser.
* Asset repository dropdown, showing "Current File" and all custom repositories.
* Display popover, filter popover and search box (partially dummies, see T82680).
* Navigation region showing asset categories.
* Asset metadata editing UI in the sidebar (preview, description, tags).
* Make asset files draggable (with preview image).
* If a repository with invalid path is selected, draw a message in the main region to help the user understand what's wrong.
* Operators to add and remove asset tags.
* "Only Assets" option for Link/Append.

Differential Revision: https://developer.blender.org/D9725

Asset Browser: Internal File Browser changes & Python API additions

The Asset Browser will be a sub-editor of the File Browser. This prepares the File Browser code for that.

**File-Lists**
* Support loading assets with metadata read from external files into the file-list.
* New main based file-list type, for the "Current File" asset repository.
* Refresh file-list when switching between browse modes or asset repositories.
* Support empty file-lists (repository with no assets).
* Store file previews as icons, so scripts can reference them via icon-id. See D9719.

**Space Data**
* Introduce "browse mode" to differeniate between file and asset browsing.
* Add `FileAssetSelectParams` to `SpaceFile`, with `FileSelectParams` as base. Makes sure data is separated between asset and file browsing when switching between them. The active params can be obtained through `ED_fileselect_get_active_params()`.
* `FileAssetSelectParams` stores the currently visible repository ID.
* Introduce file history abstraction so file and asset browsing can keep a separate history (previous and next directories).

**General**
* Option to only show asset data-blocks while file browsing (not exposed here).
* Add "active_file" context member, so scripts can get and display info about the active file.
* Add "active_id" context member, so `ED_OT_lib_id_load_custom_preview` can set a custom ID preview. (Only for "Current File" repository)
* Expose some of `FileDirEntry` in RNA as (non-editable). That way scripts can obtain name, preview icon and asset-data.

Differential Revision: https://developer.blender.org/D9724

Asset Browser: Support custom asset repository paths through Preferences

Idea is simple: In {nav Preferences > File Paths}, you can create custom repositories, give them a name and select a path.
Currently, this has to be a path to a .blend, only .blend files are supported as repositories right now. But directories should be supported as well, see {T82682}.

Differential Revision: https://developer.blender.org/D9722