always group multiple Wnck.Window of the same Wnck.Application, for all Wnck.Application.
group multiple Wnck.Window of the same Wnck.Application for some Wnck.Application, when there is not enough place to have a good-looking list of all Wnck.Window.
never group multiple Wnck.Window of the same Wnck.Application.
Type defining the policy of the Wnck.Tasklist for grouping multiple Wnck.Window of the same Wnck.Application.