R1:e00d3b72febc

Improve the implementation of Notifications

Summary:
Currently, you can't change a notification that's already shown. There's no reason for this.

(I'm planning to put file upload progress/errors in notifications.)

- Make `setContent()` and `setDuration()` immediately affect the notification.
- When there are more than 5 notifications, queue them up instead of dropping them.
- Allow arbitrarily many classes to be added/removed.
- Make the examples in the UIExamples tests more rich.

Test Plan…