WindowServer+LibGUI: Support the 4th and 5th mouse buttons

We'll call the "Back" and "Forward" since that's what they are normally
mapped to in some other systems.

Libraries/LibGUI/Event.h

@@ -257,6 +257,8 @@ enum MouseButton : u8 {
     Left = 1,
     Right = 2,
     Middle = 4,
+    Back = 8,
+    Forward = 16,
 };
 
 class KeyEvent final : public Event {