Taskbar: Integrate clock widget into taskbar window

Instead of running the clock widget as a separate menu applet,
let's just draw a clock here in the taskbar window.

Userland/Services/Taskbar/main.cpp

@@ -51,8 +51,8 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    TaskbarWindow window;
-    window.show();
+    auto window = TaskbarWindow::construct();
+    window->show();
 
     return app->exec();
 }