Improve notifications for multiple conversations in a single window

To reproduce: start conversations with multiple people, more than what fits in a single window. If you have a "<" ">" at the top that you can use to navigate between tabs, you are doing it right.

If a contact that not in the current visible tabs (conversations) messages you, there is no notification other a slight nudge on the the "<" ">" arrows. We should come up with a better way to notify users if they have multiple conversations about activity on a hidden tab/conversation.

(Reported by arma.)