Docx reader: Change behavior of Super/Subscript

In docx, super- and subscript are attributes of Vertalign. It makes more
sense to follow this, and have different possible values of Vertalign in
runStyle. This is mainly a preparatory step for real style parsing,
since it can distinguish between vertical align being explicitly turned
off and it not being set.

In addition, it makes parsing a bit clearer, and makes sure we don't do
docx-impossible things like being simultaneously super and sub.