Change the GimpDialogFactory signals "dock-added" and "dock-removed"
to "dock-window-added" and "dock-window-removed". Doing this makes
sense for a couple of reasons. First of all, the dialog factory is
built around top-levels. Second of all, the listeners to the signals
(such as the "recently closed docks" construct) work on a
gtk-window-level, not a dock level.

This change is a preparation for when GimpDock will stop being a
GimpDockWindow.