Use OpenSSL for encryption if installed

[martinrotter]

https://www.shellhacks.com/encrypt-decrypt-file-password-openssl/

It is easy to encrypt data with opensssl: echo "data" | openssl enc -aes-256-cbc -salt -k "password" and decrypt it: echo "encrypted data" | openssl enc -aes-256-cbc -salt -d -k "password".

Luckily, openssl returns non-zero return code if password is not correct.

Or we can use the library directly instead of calling external openssl executable.

https://wiki.openssl.org/index.php/EVP_Symmetric_Encryption_and_Decryption

[martinrotter]

I upgrade my Qt build to OpenSSL 1.1 branch.