All the table and cell objects know how to undo this change, what was
missing is the begin/end undo calls and the broadcast of the cell
format change.

Change-Id: I3dfd203faf5c579da2937fedab5647129a8e903a
Reviewed-on: https://gerrit.libreoffice.org/24247Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>