Blender Git Loki

Assets: Initial Asset List as part of the Asset System design

Implements a basic, WIP version of the asset list. This is needed to
give the asset view UI template asset reading and displaying
functionality.

See:
* Asset System: Data Storage, Reading & UI Access - https://developer.blender.org/T88184

Especially the asset list internals should change. It uses the
File/Asset Browser's `FileList` API, which isn't really meant for access
from outside the File Browser. But as explained in T88184, it does a lot
of the stuff we currently need, so we (Sybren St�vel and I) decided to
go this route for now. Work on a file-list rewrite which integrates well
with the asset system started in the `asset-system-filelist` branch.

Further includes:
* Operator to reload the asset list.
* New `bpy.types.AssetHandle.get_full_library_path()` function, which
gets the full path of the asset via the asset-list.
* Changes to preview loading to prevent the preview loading job to run
eternally for asset views. File Browsers have this issue too, but
should be fixed separately.