Add support for new groupchats

[subliun]

This would be useful and you'll have to do it eventually. You can just put it in a different branch. The API is here https://github.com/JFreegman/toxcore/blob/new_groupchats/toxcore/tox.h#L781-L1080

[suhr]

Sure, when they get less broken.

[zetok]

from what I've heard, they are supposed to have a sane API now
https://github.com/JFreegman/toxcore/blob/new_groupchats/toxcore/tox.h#L2290

[suhr]

Indeed they have. Will implement ~later.