[Request] Implement a way to inhibit the screen saver

[A6GibKm]

This is already covered by gamemode (See #146), but users might want to do this without gamemode when playing a game.

See the https://docs.gtk.org/gtk3/method.Application.inhibit.html method, and lutris/lutris#4150 for a reference implementation.

[mirkobrombin]

Thanks for reporting and for the documentation!