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 GNOME/gnome-music#2
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