TODO for Jabber:

- support for public channels
- implement support for transports/agents
- support for file transfers
- make sure that messages sent via a gateway from a non-subscribed contact
  are actually displayed
- allow synchronization of offline changes / online changes
- find out about Jabber server-side filtering components and if they can be
  integrated into the Kopete filtering API
- support all message types (chat/ticker/etc)
- do a std translation of resource.status().show() to JabberProtocol::Presence
- JabberProtocol and JabberContact are now friends. Move all methods that only
  relate to the API between them to private
