Bug 727762 - Undo- and redobutton in Undo History Dialog (gimpundoeditor)...

...don't work

Make action_data_get_display() return the right display also if the
action callback's user_data is a GimpImageEditor (GimpUndoEditor in
this case). The undo and redo actions now require a display, not only
an image, so this broke as a side effect.