This isn't perfect (especially the global object situation in
activate_event_handler), but I believe it's in a much more complete
state now :^)
This fixes the issue of crashing in prepare_for_ordinary_call with the
`i < m_size` crash, as it now uses the IDL callback functions which
requires the Environment Settings Object. The environment settings
object for the callback is fetched at the time the callback is created,
for example, WrapperGenerator gets the incumbent settings object for
the callback at the time of wrapping. This allows us to remove passing
in ScriptExecutionContext into EventTarget's constructor.
With this, we can now drop ScriptExecutionContext.
A legacy platform object is a non-global platform object that
implements a special operation. A special operation is a getter, setter
and/or deleter. This is particularly used for old collection types,
such as HTMLCollection, NodeList, etc.
This will be used to make these spec-compliant and remove their custom
wrappers. Additionally, it will be used to implement collections that
we don't have yet, such as DOMStringMap.
