ensure_uline_accel_group() returns the GtkAccelGroup used by GTK during the time that the calling GtkMenu is popped up, if one exists, and creates one for internal use if there is none.

This is currently the only way to get accelerators working properly under win32. It is also used internally for popup menus in GtkItemFactory. However, note that this method has been eliminated from later versions of GTK+. The replacements in GtkItemFactory are purely internal; the new methods in GtkMenu are not.