Do not set up the Application data inside the constructor
The GActions and CSS should be added inside the GApplicationClass.startup()
virtual function.
The Window instance should be created inside the GApplicationClass.activate()
virtual function.
The latter will also avoid the:
(Tour:2): Gtk-CRITICAL **: 17:42:26.102: New application windows must be added after the GApplication::startup signal has been emitted.
warning at run time.