Bubbling and Cancelling of events with Evented

[kitsonk]

Enhancement

We had discussed if Evented would have built in functionality for bubbling and cancelling (see: dojo/core#78).

We have found use cases where cancelling events is useful (see: createStateful, createCloseableMixin, and createFormFieldMixin).

We also have identified a use case for bubbling where when dealing with widgets asynchronously that emit events, it is difficult to "capture" that instances easily, making it difficult to instrument your code for dealing with errors, where if the event bubbled, in the widget's case, you could quite easily listen on the projector for errors.

I had initially been resistant to over complicate Evented until we had valid use cases, but it is clear that both of those use cases should be supportable in the base class.