Add incredibly rudimentary "chat modified" event

- Groupchats change, we get invited to them, etc. Currently absolutely none of
  this gets exposed.
- We now expose the very basic :chat-modified event that lets you know that
  *something* happened to a group chat with a given JID. Useful if you're going
  to go and re-fetch its metadata anyway, but not very useful in terms of
  figuring out what actually happened.
- This might be improved in future.