Blender Git Loki

Assets/UI: Improve asset library Preferences UI

* Open File Browser when pressing "Add Asset Library". This just makes sense,
since users have to select a directory for the asset library anyway.
* Move '+' icon back to the right side of the box. Then it is right under the
'x' icons for each indivdual library, which seems like the more natural
place.
* Correct tooltip for the "Add Asset Library" operator.
* Mark empty asset library name or paths field in red, to make clear that these
need to be set.