always group multiple #WnckWindow of the same #WnckApplication, for all #WnckApplication.
group multiple #WnckWindow of the same #WnckApplication for some #WnckApplication, when there is not enough place to have a good-looking list of all #WnckWindow.
never group multiple #WnckWindow of the same #WnckApplication.
Type defining the policy of the #WnckTasklist for grouping multiple #WnckWindow of the same #WnckApplication.