If we're using GTK+ 2.21.8 (where gtk_dialog_set_has_separator() is
deprecated but the property is still present and defaults to TRUE), we
still need to set the property to FALSE. So instead use g_object_set()
up through GTK+ 2.90.6, after which the property itself is gone.
Unfortunately the default value for this property is TRUE (bzzt, WRONG!)
so we can't just remove the function outright until we require GTK+ 2.22.
It was deprecated in GTK+ 2.21.8.
