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/HTML/HTMLBodyElement.h

@@ -41,6 +41,9 @@ public:
     virtual void apply_presentational_hints(CSS::StyleProperties&) const override;
 
 private:
+    // ^HTML::GlobalEventHandlers
+    virtual EventTarget& global_event_handlers_to_event_target() override;
+
     RefPtr<CSS::ImageStyleValue> m_background_style_value;
 };