* Fixes missing update of the catalog tree when adding catalogs.
* Adds iterators for the catalogs, needed for UI code.
* Store catalog ID in the catalog tree items, needed for UI code.
* Other smaller API additions for the UI.
* Improve comments and smaller cleanups.
New functions are covered with unit tests.
Differential Revision: https://developer.blender.org/D12618
Reviewed by: Sybren Stüvel