[wip] API changes

This decouples the _external_ API. Internally, we create a dummy
ASS_Library object to avoid changing most code. This also gives
easy backward compatibility with the old (not yet removed) API.

Mostly untested. Also a bit broken: this won't read fonts embedded
in .ass files, because ASS_Track has a different library handle.

Future directions: decouple ASS_Track from ASS_Library too.