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/TaskbarWidget.h

@@ -0,0 +1,13 @@
+#include <LibGUI/GFrame.h>
+
+class TaskbarWidget final : public GFrame {
+public:
+    TaskbarWidget(GWidget* parent = nullptr);
+    virtual ~TaskbarWidget() override;
+
+    virtual const char* class_name() const override { return "TaskbarWidget"; }
+
+private:
+    virtual void paint_event(GPaintEvent&) override;
+
+};