to abstract over css::sheet::GeneralFunction and
css::sheet::GeneralFunction2

Change-Id: I4d4df35a249bf66e12240a3b94948918f1f3ef8a
Reviewed-on: https://gerrit.libreoffice.org/35168Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>