Basic support for notification payloads in the panel.

Line-item veto is there, but allows you to cancel some
notifications you probably shouldn't be canceling. (Should
hide the "X" in those cases.)

No preference given to "sticky" notifications, because
there's no such thing yet.

Notifications are now limited to 4 visible icons, per spec.
The implementation is a total hack for now.

Change-Id: Ibdf433ae94189117f983c510fe5e0cff0bf5c44c