WindowServer+Taskbar: Show applets in taskbar :^)

WindowServer now collects applet windows into an "applet area" which is
really just a window that a WM (window management) client can position
via IPC.

This is rather hackish, and I think we should come up with a better
architecture eventually, but this brings back the missing applets since
the global menu where they used to live is gone.

Userland/Services/Taskbar/TaskbarWindow.h

@@ -53,4 +53,5 @@ private:
     NonnullRefPtr<GUI::Menu> m_start_menu;
     RefPtr<GUI::Widget> m_task_button_container;
     RefPtr<Gfx::Bitmap> m_default_icon;
+    RefPtr<GUI::Frame> m_applet_area_container;
 };