update ItemEncrypted plugin to support GPG 2.1

[realchrisolin]

This PR is a work-in-progress designed to resolve #2463. I don't know if I'll ever be able to finish it, but it should give someone more familiar with C++ and Qt that's trying to fix this issue a place to start and build on.

[hluk]

Can you explain how the fix should work? Should this just skip the --import and --export command if the gpg version is 2.1 or above?

BTW, on Linux I get version 2.4.0 and all works well:

❯ gpg --version
gpg (GnuPG) 2.4.0
libgcrypt 1.10.2-unknown
Copyright (C) 2021 Free Software Foundation, Inc.
...

I guess the check would need to include the Copyright line too.

[hluk]

Actually, I think I understand the situation from your old comment (#1208 (comment)). Let me try implementing the fix.

[hluk]

Should be fixed - see #1208 (comment).

Thanks for the help! 🙏