Blender Git Loki

Asset Catalogs: add catalog filter for the asset browser

Given an "active catalog" (i.e. the one selected in the UI), construct
an `AssetCatalogFilter` instance. This filter can determine whether an
asset should be shown or not. It returns `true` when The asset's catalog
ID is:

- the active catalog,
- an alias of the active catalog (so different UUID that maps to the
same path),
- a sub-catalog of the active catalog.

Not yet hooked up to the UI.