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.

Start by reading the notification and notifier behavior described in the issue, then review the linked EventReport pull request for the proposed event types. Compare the two suggested approaches and determine how notification matching should work for sentinel types. Done means the behavior is specified and implemented with coverage for a general change notification catching a special change.

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