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.cpp

@@ -0,0 +1,18 @@
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/Bindings/Wrapper.h>
+
+namespace Web {
+namespace Bindings {
+
+Wrappable::~Wrappable()
+{
+}
+
+void Wrappable::set_wrapper(Wrapper& wrapper)
+{
+    ASSERT(!m_wrapper);
+    m_wrapper = wrapper.make_weak_ptr();
+}
+
+}
+}