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