Document and HTMLElement now inherit from HTML::GlobalEventHandlers
which allows them to support "onfoo" event handler attributes.
These are assignable both via IDL attributes and content attributes.
Event listeners constructed this way get a special "attribute" flag
on them so we know which one to replace if you reassign them.
This also allows them to coexist with EventTarget.addEventListener().
This is all a bit sloppy, but it works decently for a first cut.
The Window object should also inherit GlobalEventHandlers, but since
we don't generate it from IDL, I haven't taken that step here.
Also this would be a lot nicer if we supported IDL mixins.
This fills in a bunch of the FIXMEs that was in prepare_script.
execute_script is almost finished, it's just missing the module side.
As an aside, let's not assert when inserting a script element with
innerHTML.
