Blender Git Loki

UI Tree-View API: Enforce active item to be un-collapsed

Makes sure that the active item of a tree never has collapsed parent
items, which can be confusing if it happens. E.g. for the asset catalogs
UI, the active catalog decides which assets are visible. Having it
hidden while being the main factor deciding which assets are visible is
quite confusing.

I think it makes sense to have this at the UI Tree-View level, rather
than doing it manually in the asset catalog code for example. Seems like
something you'd commonly want. We can make it optional in the API if
needed.

Renamed the `set_active()` function to make clear that it is more than a
mere setter.