qt: Add job for refreshing OpenPGP keys

* lang/qt/src/qgpgmerefreshopenpgpkeysjob.h,
lang/qt/src/qgpgmerefreshopenpgpkeysjob.cpp: New.
* lang/qt/src/Makefile.am: Add new files.
* lang/qt/src/protocol_p.h (Protocol::refreshKeysJob): Add support for
OpenPGP protocol.

The new job performs a --locate-external-keys for the email addresses
of all not revoked user IDs of the given keys, and it performs a
--recv-keys for the fingerprints of the given keys. The former may
import new keys from WKD or an LDAP server or via some other
(hopefully trusted) auto-key-locate mechanism.

• GnuPG-bug-id: T5951