Add new 'SwRedlineExtraData_FormattingChanges' class for formatting changes

This class is for use in the 1st step with the DOCX importer\exporter,
to hold information about 'track changes' that relate to formatting of
text portions.

Change-Id: I3f5231d0e49e0b493ec22efaac1c90b0d415f3ea
Reviewed-on: https://gerrit.libreoffice.org/6901
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>