Make GimpToolbox and GimpToolPalette use the GimpUIManager and
GimpDialogFactory of the top container instead of holding references
on their own. If they hold references on their own, we need to update
these when we move e.g. GimpToolbox between different top level
containers such as GimpDockWindow and GimpImageWindow. Failure to do
this cased problems talked about in bug 646794 (case 2).