Define Notification Types With Sentinels?

[rmorshea]

A notification which catches a "change" should also intuitively catch a "special change".

I see two ways to accomplish this kind of behavior:

1. Split type strings at each " " similarly to how html classes are handled by css selectors.
2. Key notifiers on type(sentinel) and trace type(sentinel).mro() when triggering them.

Either way, I think that we could leverage EventReport types to do this.