Libanki maintenance

Catch up on changes made to libanki.

Two things:
 1 - A couple of these are for methods that are still unused in AnkiDroid, so there should be no impact.
 2 - I've skipped two changes that are more suitable for larger commits in the future. I *might* get around to doing [this one](https://github.com/dae/anki/commit/cf801e4fb4c9792347192c8c78c09bcac652ba47) for the next release, but it's not crucial.