An easy way would be to just extend aXML_WritingDirection_Enum, but then
we would write the new attribute value to a non-extension namespace.

So special case the new attribute value during both import and export
(and only for table cells as a start).

Change-Id: I431bf99693c4a3452e91f241bd2f0fcfc72c68fd
Reviewed-on: https://gerrit.libreoffice.org/67770Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins