LibWeb+LibJS: Make the EventTarget hierarchy (incl. DOM) GC-allocated

This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.

There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.

Userland/Libraries/LibWeb/CSS/Screen.cpp

@@ -12,7 +12,7 @@
 namespace Web::CSS {
 
 Screen::Screen(HTML::Window& window)
-    : RefCountForwarder(window)
+    : m_window(JS::make_handle(window))
 {
 }