improve the existing 'find unused RID constants' script

Change-Id: I6facbf9ef929bd31dc59eba4a1807c72b87cdb2f
Reviewed-on: https://gerrit.libreoffice.org/35186Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>