Now that we have RTTI in userspace, we can do away with all this manual
hackery and use dynamic_cast.
We keep the is<T> and downcast<T> helpers since they still provide good
readability improvements. Note that unlike dynamic_cast<T>, downcast<T>
does not fail in a recoverable way, but will assert if the object being
casted is not a T.
Specification: https://dom.spec.whatwg.org/#concept-event-dispatch
This also introduces shadow roots due to it being a requirement of
the event dispatcher.
However, it does not introduce the full shadow DOM, that can be
left for future work.
This changes some event dispatches which require certain attributes
to be initialised to a value.
