LibWeb: Basic implementation of global event handlers :^)

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.

Userland/Libraries/LibWeb/DOM/EventTarget.h

@@ -61,6 +61,7 @@ public:
         NonnullRefPtr<EventListener> listener;
     };
 
+    Vector<EventListenerRegistration>& listeners() { return m_listeners; }
     const Vector<EventListenerRegistration>& listeners() const { return m_listeners; }
 
     Function<void(const Event&)> activation_behaviour;