Blender Git Loki

Asset Browser: Move Asset Library selector to navigation bar

The menu to select the active Asset Library is now in the left bar (so called
"Source List", although I'd prefer "Navigation-Bar").

This has some benefits:
* All Asset Library navigation is in the left sidebar now, giving nice grouping
and a top-to-bottom & left-to-right flow of the layout. The header is focused
on view set-up now.
* Catalogs are stored inside the asset library. Makes sense to have them right
under that.
* Less content in the header allows for less wide Asset Browsers without
extensive scrolling.
* This location gives more space to add options or operators for Asset
Libraries.

Main downside I see is that the side-bar needs to be opened to change
libraries, which takes quite some space. In practice there shouldn't be need to
do this often though.