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
62cb5c5c4a