Blender Git Loki

Enable Asset Browser by default for poses, rest stays experimental

Idea for 3.0 is to disable all functionality that isn't well polished
and focus on those parts first. Starting with poses.

* Adds a new experimental option "Extended Asset Browser", replacing
"Asset Browser".
* Unlike the previous option, this isn't enabled by default anymore.
This didn't work well in practice and caused plenty of confusion.
* "Mark as Asset" and "Clear Asset" are hidden if the option is
disabled.
* Same for the category selection in the Asset Browser.
* Always show display the "Only Assets" option in the File Browser while
browing inside .blend files. That way you can hide data-blocks that
are not pose assets.
* The Asset Library setup UI in the Preferences is always visible now,
it's needed for pose library access.

Addresses T90181, T90180 and T90300.

Differential Revision: https://developer.blender.org/D12120