sw: add InteropGrabBag for character styles

offapi already documented this, but marked as optional, and previously
only paragraph styles implemented it. Do the same for character styles.

Change-Id: I3e331eb43f325ac8963984b2460db53f1be497a6
DOCX filter: handle char style's qFormat, rsid and friends

Change-Id: Ie0bc9aff722d87fcd246d0cf1971ad6f34f0996a