LibWeb: Move event listeners, handlers and callbacks to the GC heap

This patch moves the following things to being GC-allocated:
- Bindings::CallbackType
- HTML::EventHandler
- DOM::IDLEventListener
- DOM::DOMEventListener
- DOM::NodeFilter

Note that we only use PlatformObject for things that might be exposed
to web content. Anything that is only used internally inherits directly
from JS::Cell instead, making them a bit more lightweight.

Userland/Libraries/LibWeb/DOM/EventDispatcher.h

@@ -18,7 +18,7 @@ public:
 
 private:
     static void invoke(Event::PathEntry&, Event&, Event::Phase);
-    static bool inner_invoke(Event&, Vector<NonnullRefPtr<DOM::DOMEventListener>>&, Event::Phase, bool);
+    static bool inner_invoke(Event&, Vector<JS::Handle<DOM::DOMEventListener>>&, Event::Phase, bool);
 };
 
 }