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/Bindings/WindowObject.cpp

@@ -67,6 +67,8 @@ void WindowObject::initialize()
 {
     GlobalObject::initialize();
 
+    set_prototype(&ensure_web_prototype<EventTargetPrototype>("EventTarget"));
+
     define_property("window", this, JS::Attribute::Enumerable);
     define_property("frames", this, JS::Attribute::Enumerable);
     define_property("self", this, JS::Attribute::Enumerable);