LibWeb: Add a DOM Event class (instead of events being simple strings)

This patch adds the Event base class, along with a MouseEvent subclass.
We now dispatch MouseEvent objects for mousedown, mouseup and mousemove
and these objects have the .offsetX and .offsetY properties.

Both of those properties are hard-coded at the moment. This will be
fixed in the next patch. :^)

Libraries/LibWeb/Forward.h

@@ -31,21 +31,25 @@ namespace Web {
 class CanvasRenderingContext2D;
 class Document;
 class Element;
+class Event;
 class EventListener;
 class EventTarget;
 class Frame;
 class HTMLElement;
 class HTMLCanvasElement;
 class HtmlView;
+class MouseEvent;
 class Node;
 
 namespace Bindings {
 
 class CanvasRenderingContext2DWrapper;
 class DocumentWrapper;
+class EventWrapper;
 class EventListenerWrapper;
 class EventTargetWrapper;
 class HTMLCanvasElementWrapper;
+class MouseEventWrapper;
 class NodeWrapper;
 class Wrappable;
 class Wrapper;