Rename to Notification
Eliminate done event.

Update helper menthods to:
OverlayCloser openNotification(MessageStyleType type, int timeout_msec, View viewToShow);
OverlayCloser openNotification(MessageStyleType type, int timeout_msec, String title);

Add
OverlayCloser openNotification(MessageStyleType type, int timeout_msec, String title, String linkText, NotificationCallback cb);

NotificationCallback is for when users click on the link.