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 Stüvel 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.
Allow users to undo the effect of the "Generate Preview" operator in the
asset browser (`ED_OT_lib_id_generate_preview`). Without this, the
button is too dangerous.
It seems generally preferred to have new files be created with C++.
The only reason I didn't do that when I initially created the files is that I
was unsure about some C-API aspect.
Also use nullptr instead of NULL.
