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.cpp

@@ -0,0 +1,29 @@
+#include "TaskbarWindow.h"
+#include "TaskbarWidget.h"
+#include <LibGUI/GWindow.h>
+#include <LibGUI/GDesktop.h>
+#include <stdio.h>
+
+TaskbarWindow::TaskbarWindow()
+{
+    set_window_type(GWindowType::Taskbar);
+    set_title("Taskbar");
+    set_should_exit_event_loop_on_close(true);
+
+    on_screen_rect_change(GDesktop::the().rect());
+
+    GDesktop::the().on_rect_change = [this] (const Rect& rect) { on_screen_rect_change(rect); };
+
+    auto* widget = new TaskbarWidget;
+    set_main_widget(widget);
+}
+
+TaskbarWindow::~TaskbarWindow()
+{
+}
+
+void TaskbarWindow::on_screen_rect_change(const Rect& rect)
+{
+    Rect new_rect { rect.x(), rect.bottom() - taskbar_height() + 1, rect.width(), taskbar_height() };
+    set_rect(new_rect);
+}