app: Make GimpToolbox use parent UI manager
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).
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