Blender Git Loki

Cleanup: Centralize/unify asset library reference from/to enum code

This was an open TODO, I wanted to have code for translating asset
library references from and to enum values in a central place, and
access that in the same way from both the Asset Browser and the
Workspace RNA code.

* Adds own file for the related functions.
* Adds doxygen comments.
* Updates RNA callbacks to properly use these functions.
* Let these functions call each other, avoid duplicating logic.