Use gtk_window_has_group() to know if the window has an explicit window group.

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