Add support for text/pgp.
Summary:
If Someone sends an encrypted attachment without any content, we should
display the content of the encrypted attachment.
Test Plan:
show one text/pgp encrypted attachment correctly (included as testdata).
Also make sure, that a "fake" text/pgp attachment don't break the viewer.
This part is not working at the moment, I'll look at it later...
Reviewers: KDE PIM, vkrause
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8521