Add a 'new messages' indicator in messages list
When the user is reading old messages and a new one arrives for that chat/IM, we don't scroll down, since it is annoying. But we should add an indication that new message(s) are unread in this thread. Something visible, but that doesn't interrupt the history reading. The indicator should follow the user scrolling all over the history until: he reaches the oldest of the new messages or he clicks on the indicator, that should scroll down to that position.