Add support for notification menu (with badges)

[job13er]

To this end, we should refactor the existing notifier service to support adding both notifications (things that go into the notification menu) and transient messages (what we currently call notifications).

I propose we change the current addNotification() interface to be a more accurate showMessage() interface, and use addNotification() for adding notifications to the notification menu (updating the badge count if the menu isn't visible).