LibWeb: Generate Event and MouseEvent bindings from IDL :^)

We still have to hand-write a function to turn an Event& into a wrapper
but this is still a hue improvement. Eventually we'll find a way to
auto-generate that function as well.

Libraries/LibWeb/DOM/MouseEvent.idl

@@ -0,0 +1,6 @@
+interface MouseEvent : Event {
+
+    readonly attribute double offsetX;
+    readonly attribute double offsetY;
+
+}