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:

• Verified normal notifications continue to function as expected.
• Played with the UIExamples notifications:
  • Verified the "update every second" notification udpated every second.
  • Verified the permanent alert notification was yellow and requires a click to dismiss.
  • Verified the interactive notification responds correctly to "OK" / "Cancel".
  • Verified the "click every 2 seconds" notification doesn't vanish until not clicked for 2 seconds.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3653