Define Notification Types With Sentinels?

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`](https://github.com/ipython/traitlets/pull/251) types to do this.