Turns the asset shelf into a popover which reduces some of the special
handling. An operator `WM_OT_call_asset_shelf_popover()` (similar to
`WM_OT_call_panel()`) is added to be able to call the popover from shortcuts.
Exactly this was an important aspect for the brush assets project, to allow
quick searching for brushes from the popup.
A custom shortcut can be added to asset shelf popovers using "Assign Shortcut"
in the context menu of buttons invoking it.
The popover is spawned with the mouse hovering the first asset and the search
button active using "semi modal" handling. That means while the popover is
open, any text input is captured by the search button, while the rest of the
popover stays interactive. So for example navigating through asset catalogs is
possible, a single click activates an asset and closes the popover.
Reviewed as part of the asset shelf project, see:
- https://projects.blender.org/blender/blender/issues/116337
- https://projects.blender.org/blender/blender/pulls/106303
721dbfccfd