implement instant-messaging notifications

[foudfou]

asked by rmtk

Some preliminary remarks:

• it should be easy to be notified from "new messages" (observe topic== 'unread-im-count-changed', see http://mxr.mozilla.org/comm-central/source/mail/components/im/content/chat-messenger-overlay.js).
• "new messages" = private messages or channels where my nick appears
• don't know how to be notified from user status changes (apart from topic== 'new-text')
• we'll probably have to use a second icon for IM, for keeping track of new mail messages and new IM messages at the same time