[HTML] Add a common HTML event handler subsystem

(EventListenerImp): Keep an HTML event handler separately in functionObject; all callers changed.
(EventListenerImp::getEventHandler, EventListenerImp::setEventHandler): New
(EventTargetImp): Keep at most one listener for an HTML event handler in each Listener list
(EventTargetImp::getEventHandlerListener, EventTargetImp::getEventHandler, EventTargetImp::setEventHandler): New
(ECMAScriptContext::dispatchEvent): Refine to use the HTML event handler stored in EventListenerImp
(DocumentImp, HTMLElementImp, WindowImp) : Update all event handler IDL attributes
(DocumentWindow::setEventHandler) : New
(DocumentImp::setEventHandler) : New
(HTMLElementImp::setEventHandler) : New
testdata/html-001.html : New