look for virtual methods where all of the overrides of the method are
empty

Change-Id: I87d99a0b647700a8d53498e0ab5f0437d3508553
Reviewed-on: https://gerrit.libreoffice.org/54060Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>