Set reference to roster contact on the message and chatbox object and
listen for changes to the nickname.

Currently, because chat boxes are fetched and initialized before the
roster, messages and chats are repainted with the correct display name
only later, causing a "flash" effect.

Ideally we would only initialize the chat boxes after the roster
contacts have been fetched, but this is currently not easily possible
because we need the control box to render before everything else.