Use libnotify for challenge codes

[micycle8778]

When you use onlykey-gpg-agent, the challenge codes are outputted to the terminal, which makes it inconvientent to alt-tab to another terminal to use GPG. It would help the UX if these codes were provided through a notifcation.

[onlykey]

Yes, this is one of the things we still want to implement. We are looking for a way to support this on both Linux and Mac though, eventually Windows too.

You can also disable the challenge code and only require pressing any button on OnlyKey.