Blender Git Loki

Asset Browser: Initial Asset Catalog UI

The Asset Browser now displays a tree with asset catalogs in the left
sidebar.
This replaces the asset categories. It uses the new UI tree-view API
(https://wiki.blender.org/wiki/Source/Interface/Views#Tree-View).
Buttons are displayed for adding and removing of catalogs. Parent items
can be collapsed, but the collapsed/uncollapsed state is not stored in
files yet.
Note that edits to catalogs (e.g. new or removed catalogs) are only
written to the asset library's catalog definition files when saving a
.blend.

In the "Current File" asset library, we try to show asset catalogs from
a parent asset library, or if that fails, from the directory the file is
stored in. See adaf4f56e1ed.

There are plenty of TODOs and smaller glitches to be fixed still. Plus a
UI polishing pass should be done.

Important missing UI features:
* Dragging assets into catalogs (WIP, close to being ready).
* Renaming catalogs
* Proper handling of catalogs in the "Current File" asset library
(currently not working well).

The "Current File" asset library is especially limited still. Since this
is the only place where you can assign assets to a catalog, this makes
the catalogs very cumbersome in general. To assign an asset to a
catalog, one has to manually copy the Catalog ID (a random hash like
number) to the asset metadata through a temporary UI in the Asset
Browser Sidebar. These limitations should be addressed over the next few
days, they are high priority.

Differential Revision: https://developer.blender.org/D12670