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/NodeWrapper.h

@@ -43,5 +43,7 @@ private:
     virtual const char* class_name() const override { return "NodeWrapper"; }
 };
 
+NodeWrapper* wrap(JS::Heap&, Node&);
+
 }
 }