Blender Git Loki

Asset Catalogs: support reloading without losing local changes

Keep track of unsaved asset catalog changes, in a more granular way than
just one boolean per asset library. Individual catalogs can now be
marked with a flag `has_unsaved_changes`. This is taken into account
when reloading data from the catalog definition file (CDF):

- New catalog in CDF: gets loaded
- Already-known catalog in CDF:
- local unsaved changes: on-disk catalog is ignored
- otherwise: on-disk catalog replaces in-memory one
- Already-known catalog that does not exist in CDF:
- local unsaved changes: catalog is kept around
- otherwise: catalog is deleted.

Because this saving-is-also-loading behaviour, the "has unsaved changes"
flags are all stored in the undo buffer; undoing after saving will not
change the CDF, but at least it'll undo the loading from disk, and it'll
re-mark any changes as "not saved".

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D12967