Added new helper class to svx for numbering

Change-Id: Ie7db4059568024e9bed460fcf7c9215bd2ef788f
Reviewed-on: https://gerrit.libreoffice.org/26020Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>