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. :^)

LibGUI/GDesktop.h

@@ -2,6 +2,7 @@
 
 #include <AK/AKString.h>
 #include <AK/Badge.h>
+#include <AK/Function.h>
 #include <SharedGraphics/Rect.h>
 
 class GEventLoop;
@@ -17,6 +18,8 @@ public:
     Rect rect() const { return m_rect; }
     void did_receive_screen_rect(Badge<GEventLoop>, const Rect&);
 
+    Function<void(const Rect&)> on_rect_change;
+
 private:
     Rect m_rect;
 };