Taskbar: Add a quick launch bar

This is a tiny bar at the left of the taskbar where you can put
your most used apps to launch them with a single click. In a way,
it's another replacement for the Launcher, in addition to the app
menu. Unlike the launcher and the menu, it's not meant to be the
primary way to launch apps; it's only a faster way to launch a few
most often used utilities.

Applications/Taskbar/TaskbarWindow.h

@@ -11,6 +11,7 @@ public:
     int taskbar_height() const { return 28; }
 
 private:
+    void create_quick_launch_bar();
     void on_screen_rect_change(const Rect&);
     NonnullRefPtr<GButton> create_button(const WindowIdentifier&);