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/DOM/EventTarget.h

@@ -45,7 +45,7 @@ public:
 
     void add_event_listener(String event_name, NonnullRefPtr<EventListener>);
 
-    virtual void dispatch_event(String event_name) = 0;
+    virtual void dispatch_event(NonnullRefPtr<Event>) = 0;
 
     struct EventListenerRegistration {
         String event_name;