Add tokens for 'table cell insert & delete' in DOCX's model.xml

Add appropriate tokens for importing the 'table cell insert'
and 'table cell delete' token in the DOCX's model.xml file.

Change-Id: I328dfde1cef42f1c61ea0c481552f07ea7683def
Reviewed-on: https://gerrit.libreoffice.org/7871
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>