window.py : Replace deprecated 'add_accelerator()' with 'set_accels_for_action()' method

Gtk.Application.add_accelerator(accelerator, action_name, parameter) used to set accelerator for some action is deprecated since Gtk version 3.14.

The recommended method is Gtk.Application.set_accels_for_action(detailed_action_name, accels).

For reference : https://lazka.github.io/pgi-docs/Gtk-3.0/classes/Application.html#Gtk.Application.add_accelerator.

Fixes #2 (closed)