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/SVGElement.cpp

@@ -4,14 +4,23 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/SVGElementPrototype.h>
+#include <LibWeb/HTML/Window.h>
 #include <LibWeb/SVG/SVGElement.h>
 
 namespace Web::SVG {
 
 SVGElement::SVGElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : Element(document, move(qualified_name))
-    , m_dataset(JS::make_handle(HTML::DOMStringMap::create(*this)))
+    , m_dataset(HTML::DOMStringMap::create(*this))
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::SVGElementPrototype>("SVGElement"));
+}
+
+void SVGElement::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_dataset.ptr());
 }
 
 }