Assets: Add an active asset library per workspace, for the UI to use

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.

source/blender/blenloader/intern/versioning_300.c

@@ -33,9 +33,11 @@
 #include "DNA_listBase.h"
 #include "DNA_modifier_types.h"
 #include "DNA_text_types.h"
+#include "DNA_workspace_types.h"
 
 #include "BKE_action.h"
 #include "BKE_animsys.h"
+#include "BKE_asset.h"
 #include "BKE_collection.h"
 #include "BKE_deform.h"
 #include "BKE_fcurve_driver.h"
@@ -517,6 +519,15 @@ void blo_do_versions_300(FileData *fd, Library *UNUSED(lib), Main *bmain)
       }
     }
     FOREACH_NODETREE_END;
+
+    {
+      if (!DNA_struct_elem_find(
+              fd->filesdna, "WorkSpace", "AssetLibraryReference", "active_asset_library")) {
+        LISTBASE_FOREACH (WorkSpace *, workspace, &bmain->workspaces) {
+          BKE_asset_library_reference_init_default(&workspace->active_asset_library);
+        }
+      }
+    }
   }
 
   if (!MAIN_VERSION_ATLEAST(bmain, 300, 10)) {