Add signing and verifying capabilities in QMF, contributes to MER#1780
This MR introduces a plugin framework in QMF to handle cryptographic operations. Currently sign and verify are defined as an interface, but later on, encryption and decryption may be added. The crypto interface is implemented by GPGME and works for pgp-application multipart/signed types, S/MIME may be introduced later.
I've added a test program as @mvogt suggested, but the sign part is not working because of to old versions of GnuPG, not properlly handling the gpgme_set_passphrase_cb() call.
This MR is related to gnupg2!5 (merged) and dcaliste/gpgme#1.