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/Forward.h

@@ -28,19 +28,24 @@
 
 namespace Web {
 
+class CanvasRenderingContext2D;
 class Document;
 class Element;
 class EventListener;
 class EventTarget;
 class Frame;
+class HTMLElement;
+class HTMLCanvasElement;
 class HtmlView;
 class Node;
 
 namespace Bindings {
 
+class CanvasRenderingContext2DWrapper;
 class DocumentWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;
+class HTMLCanvasElementWrapper;
 class NodeWrapper;
 class Wrappable;
 class Wrapper;