LibWeb: Add <canvas> element and start fleshing out CRC2D

This patch adds HTMLCanvasElement along with a LayoutCanvas object.
The DOM and layout parts are very similar to <img> elements.

The <canvas> element holds a Gfx::Bitmap which is sized according to
the "width" and "height" attributes on the element.

Calling .getContext("2d") on a <canvas> element gives you a context
object that draws into the underlying Gfx::Bitmap of the <canvas>.
The context weakly points to the <canvas> which allows it to outlive
the canvas element if needed.

This is really quite cool. :^)

Libraries/LibWeb/Bindings/Wrappable.h

@@ -46,7 +46,7 @@ private:
 };
 
 template<class NativeObject>
-inline Wrapper* wrap(JS::Heap& heap, NativeObject& native_object)
+inline Wrapper* wrap_impl(JS::Heap& heap, NativeObject& native_object)
 {
     if (!native_object.wrapper())
         native_object.set_wrapper(*heap.allocate<typename NativeObject::WrapperType>(native_object));