This will be inherited by documents and workers, to provide a common
abstraction for script execution. (We don't have workers yet, but we
might as well make this little space for them now to simplify things
down the road.)
For now, the new DOM::EventDispatcher is very simple, it just iterates
over the set of listeners on an EventTarget and invokes the callbacks
as it goes.
This simplifies EventTarget subclasses since they no longer have to
implement the callback mechanism themselves.
