LibWeb: Make the Window object "inherit" from EventTarget :^)

Since Web::Bindings::WindowObject inherits from JS::GlobalObject, it
cannot also inherit from Web::Bindings::EventTargetWrapper.

However, that's not actually necessary. Instead, we simply set the
Window object's prototype to the EventTargetPrototype, and add a little
extra branch in the impl_from() function that turns the JS "this" value
into a DOM::EventTarget*.

With this, you can now call window.addEventListener()! Very cool :^)

Fixes #4758.

Userland/Libraries/LibWeb/DOM/XMLHttpRequest.cpp

@@ -113,7 +113,7 @@ bool XMLHttpRequest::dispatch_event(NonnullRefPtr<DOM::Event> event)
     return DOM::EventDispatcher::dispatch(*this, move(event));
 }
 
-Bindings::EventTargetWrapper* XMLHttpRequest::create_wrapper(JS::GlobalObject& global_object)
+JS::Object* XMLHttpRequest::create_wrapper(JS::GlobalObject& global_object)
 {
     return wrap(global_object, *this);
 }