In part of a table shape is selected, then only operate on the selected
cells, not on all of them.

Change-Id: I3a9ba2b99bcaa2e355b6fcdafdd142d4a809bce6
Reviewed-on: https://gerrit.libreoffice.org/33524Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>