Configurable message and presence notifications

Notifications are currently absent from the core (except the minimalistic "New Message" icon in the toolbar) and handled by scriptlets. This blueprint proposes to:

* categorize notification outputs (e.g. sound, toolbar icons, desktop popup, status bar, script)

* integrate them into the core

* provide GUI to configure what events should trigger notifications; GUI could be both contextual (e.g. right-click on contact and select "Warn when this contact is online") and centralized (e.g. graphically build a query that says "Display popup when someone is writing to me, but only if I'm not on busy state")