Blender Git Loki

Assets: Read catalogs immediately when loading a library

Until now, the asset catalogs would only show up after all assets from
the library were loaded. Now the catalogs are read first, which makes
them appear pretty much immediately. This makes the UI more responsive
and feel less heavy.

I added a dedicated file-list type for asset libraries now. While not
necessarily needed, I prefer that so asset library specific stuff can be
handled in there.