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/CSSRule.cpp

@@ -7,13 +7,13 @@
  */
 
 #include <LibWeb/Bindings/CSSRulePrototype.h>
-#include <LibWeb/Bindings/WindowObject.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
+#include <LibWeb/HTML/Window.h>
 
 namespace Web::CSS {
 
-CSSRule::CSSRule(Bindings::WindowObject& window_object)
+CSSRule::CSSRule(HTML::Window& window_object)
     : PlatformObject(window_object.ensure_web_prototype<Bindings::CSSRulePrototype>("CSSRule"))
 {
 }