Use gtk_window_has_group() to know if the window has an explicit window group.
gtk_window_get_group() never returns NULL; if the window isn't in a group, a default window group is returned instead. Use gtk_window_has_group() instead. This fixes some previous commits to use accessors to access GtkWindow. Reported by Philip Withnall in bug https://bugzilla.gnome.org/show_bug.cgi?id=627828
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