Make default OpenPGP file extension configurable
This makes it configurable if OpenPGP encrypted files created by Kleopatra should have the .pgp ending or the .gpg ending. The current behavior stays as default. (.gpg) This change was requested by instutional users who want to share files with the same ending between PGP and GnuPG and use the same file extension to mark those files. Patch adapted from commit: 1438ec3