Taskbar: Start working on a taskbar app.

I originally thought I would do this inside WindowServer, but let's try to
make it as a standalone app that communicates with WindowServer instead.
That will allow us to use LibGUI. :^)

Servers/WindowServer/WSClientConnection.h

@@ -37,6 +37,7 @@ public:
     template<typename Callback> void for_each_window(Callback);
 
     void notify_about_new_screen_rect(const Rect&);
+    void post_paint_request(const WSWindow&, const Rect&);
 
 private:
     virtual void on_message(const WSMessage&) override;