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/SVGSVGElement.cpp

@@ -6,6 +6,7 @@
  */
 
 #include <LibGfx/Painter.h>
+#include <LibWeb/Bindings/SVGSVGElementPrototype.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/StyleComputer.h>
 #include <LibWeb/DOM/Document.h>
@@ -20,6 +21,7 @@ namespace Web::SVG {
 SVGSVGElement::SVGSVGElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : SVGGraphicsElement(document, qualified_name)
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::SVGSVGElementPrototype>("SVGSVGElement"));
 }
 
 RefPtr<Layout::Node> SVGSVGElement::create_layout_node(NonnullRefPtr<CSS::StyleProperties> style)