Add support for event propagation

[justinfagnani]

I need to emulate part of the DOM along with event dispatch and propagation. While the DOM EventTarget class doesn't support this as a public interface, it would be great to be able to use this shim and provide the EventTarget class with a parent reference so that the dispatch algorithm could be implemented.

To keep from creating a non-standard interface on EventTarget itself there could be a utility function like setEventTargetParent() which sets an internal field.