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/SVG/SVGDefsElement.cpp

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/SVGDefsElementPrototype.h>
+#include <LibWeb/HTML/Window.h>
 #include <LibWeb/SVG/SVGDefsElement.h>
 
 namespace Web::SVG {
@@ -11,6 +13,7 @@ namespace Web::SVG {
 SVGDefsElement::SVGDefsElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : SVGGraphicsElement(document, move(qualified_name))
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::SVGDefsElementPrototype>("SVGDefsElement"));
 }
 
 SVGDefsElement::~SVGDefsElement()