timeline.tx: reimplement the code using Web Notifications with bb.Notification object.
timeline.tx: reimplement the code for title notification with bb.Notification object.
Add 'enable_timeline_web_notifications' per-timeline config parameter to control whether Web Notifications are enabled or not.
busybird.js, timeline.tx: Add 'subtitle' arg for bb.Notification#showWebNotification(). Now timeline name is included in the notification.