This should be less fragile and re-uses some xmloff and unocore code.

Storing from ~SwBaseNumRules() does not work because other gobals that
the export code relies on are already dead at the time it is called from
a terminate() listener, so store when the dialog updates a format.

Change-Id: I4e148b82d0a338ec7ffa6429c6e162db79f8c44e