Support DOCX exporting 'Track Changes - Inserted Paragraph Mark'

This patch adds support for the DOCX exporter to export the
'Track Changes' of type 'Inserted Paragraph Marker'.

Change-Id: Iaab42c1b678bb83832f519a2f2a8fbc42b9d729d
Reviewed-on: https://gerrit.libreoffice.org/7128
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>