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/TaskbarWindow.h

@@ -0,0 +1,16 @@
+#include <LibGUI/GWindow.h>
+#include <LibGUI/GWidget.h>
+
+class TaskbarWindow final : public GWindow {
+public:
+    TaskbarWindow();
+    virtual ~TaskbarWindow() override;
+
+    int taskbar_height() const { return 20; }
+
+    virtual const char* class_name() const override { return "TaskbarWindow"; }
+
+private:
+    void on_screen_rect_change(const Rect&);
+
+};