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. :^)

Applications/Taskbar/main.cpp

@@ -0,0 +1,10 @@
+#include <LibGUI/GApplication.h>
+#include "TaskbarWindow.h"
+
+int main(int argc, char** argv)
+{
+    GApplication app(argc, argv);
+    TaskbarWindow window;
+    window.show();
+    return app.exec();
+}