LibWeb: Start implementing basic JavaScript DOM bindings

This patch introduces the Wrapper and Wrappable classes.

Node now inherits from Wrappable, and can be wrapped in a GC-allocated
Bindings::NodeWrapper object. The only property we expose right now is
the very simple nodeName property.

When a Document's JS::Interpreter is first instantiated, we add a
"document" property with a DocumentWrapper object to the global object.

This is pretty cool! :^)

Libraries/LibWeb/Bindings/Wrappable.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <AK/WeakPtr.h>
+#include <LibJS/Heap.h>
+#include <LibWeb/Forward.h>
+
+namespace Web {
+namespace Bindings {
+
+class Wrappable {
+public:
+    virtual ~Wrappable();
+
+    void set_wrapper(Wrapper&);
+    Wrapper* wrapper() { return m_wrapper; }
+    const Wrapper* wrapper() const { return m_wrapper; }
+
+private:
+    WeakPtr<Wrapper> m_wrapper;
+};
+
+template<class NativeObject>
+inline Wrapper* wrap(JS::Heap& heap, NativeObject& native_object)
+{
+    if (!native_object.wrapper())
+        native_object.set_wrapper(*heap.allocate<typename NativeObject::WrapperType>(native_object));
+    return native_object.wrapper();
+}
+
+}
+}