Add XML dumping of the new 'SwTableCellRedline'

Change-Id: I4072130ff0ce712f02d1dd8f12003170116216a2
Reviewed-on: https://gerrit.libreoffice.org/7876
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>