Make GimpDock be a GtkVBox instead of a GimpDockWindow. This means we
can now put a GimpDock anywhere, including inside an image window.

In order to do this we need to:

 * Separate dock and dock window creation in the dialog factory and
   add a couple of new dock window constructors

 * Change gimp_dialog_factory_dock_new() to not only create a dock,
   but also create a dock window and then combine those two

 * Change the dock constructor to take a GimpUIManager since they
   depend on that during their construction. We get the ui manager
   from the dock window, but we can't create the dock *inside* the
   dock window, we have to add the dock later. So we create the dock
   window first and then pass its ui manager to the dock constructors

 * Make some other minor adaptions, mostly with
   gimp_dock_window_from_dock() and gimp_dock_window_get_dock()