Asset Browser: Show disabled-hint when dragging external assets over catalog

There's now a message displayed in red next to the cursor explaining that only
assets from the current file can be moved between catalogs.

The previous commit prepared this.

source/blender/editors/space_file/asset_catalog_tree_view.cc

@@ -389,6 +389,8 @@ bool AssetCatalogDropController::has_droppable_item(const wmDrag &drag,
       return true;
     }
   }
+
+  *r_disabled_hint = "Only assets from this current file can be moved between catalogs";
   return false;
 }