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.
Asset Browser UI:
| Demo (a bit old, UI changed slightly since then): | Threaded generation of object previews: |


