This per-workspace active asset library will be used by the asset views
later. Note that Asset Browsers have their own active asset library,
overriding the one from the workspace.
As part of this the `FileSelectAssetLibraryUID` type gets replaced by
`AssetLibraryReference` which is on the asset level now, not the
File/Asset Browser level. But some more work is needed to complete that,
which is better done in a separate commit.
This also moves the asset library from/to enum-value logic from RNA to
the editor asset level, which will later be used by the asset view.
It seems generally preferred to have new files be created with C++.
The only reason I didn't do that when I initially created the files is that I
was unsure about some C-API aspect.
Also includes some minor C++ related cleanup (nullptr instead of NULL, remove
redundant `struct` keyword).
