Document EventDispatcher changes

[wouterj]

• The Debug\TraceableEventDispatcher should be documented in components/event_dispatcher (just like the other, already documented, dispatchers)
• Document integration with DI component using the RegisterListenerPass (just a small section/sidebar in components/event_dispatcher/introduction)

[weaverryan]

The first is done, we just need to tweak some namespaces for 2.5 with it (see #3916).

[wouterj]

Thanks @xabbuh for your quick actions on this one!

[xabbuh]

@wouterj Can you reopen the issue? The second task isn't done yet.

[wouterj]

Oh, indeed. Sorry about that, ryan said everything was fixed after your last PR, but he only meant the first item :)

[xabbuh]

Never mind. :) I think we can get it fixed at the next weekend.