Uses the additions to the UI tree-view API from the previous commit to enable drag & drop of asset catalogs. The catalogs will be moved in the tree including children. A remaining issue is that a catalog with children will always be collapsed when dropping. I need to find a way to fix that in the tree-view API. There are a few improvements I can think of for the tree-item drag & drop support, but time for these is too short. These can be done as normal cleanups at some point.