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/Layout/Node.cpp

@@ -600,4 +600,29 @@ RefPtr<Painting::Paintable> Node::create_paintable() const
     return nullptr;
 }
 
+bool Node::is_anonymous() const
+{
+    return !m_dom_node.ptr();
+}
+
+DOM::Node const* Node::dom_node() const
+{
+    return m_dom_node.ptr();
+}
+
+DOM::Node* Node::dom_node()
+{
+    return m_dom_node.ptr();
+}
+
+DOM::Document& Node::document()
+{
+    return *m_document;
+}
+
+DOM::Document const& Node::document() const
+{
+    return *m_document;
+}
+
 }