In addition to "always" and "never", gnome-panel supported an "auto"
grouping mode, which only started to group items when running out of
available space. It makes sense for us to support the same option in
the window-list extension, so implement it.
https://bugzilla.gnome.org/show_bug.cgi?id=697157
When setting GtkRadioButton's group property, its active property is
also set as a by-product. This means that setting these properties
isn't commutative which arguably is a bug in gtk+ but one that we can
easily work around by just switching the order here.
https://bugzilla.gnome.org/show_bug.cgi?id=697495