Automatic key import / export + Expected monad

Summary:
There are many things going on here (perhaps a bit much for a single patch):

- When an attachment is of mime type "application/pgp-keys", a button is added to import the key to GPG
- When sending a mail and crypto is enabled (encryption, signing or both), the public key of the first private key found is sent as an un-encrypted attachment (T6994)
- The `mailcrypto.{h,cpp}` was, for the most part, rewritten
- Introduction of the expected monad, inspired by what was proposed for C++ [here](https://isocpp.org/files/papers/n4015.pdf), but not at all a strict implementation of this specification. We may want to add some more features of this standard later.

The rationale for some of the choices:

- I found mailcrypto a bit hard to edit to add new features, and a great part was commented code to prepare for the support the SMIME crypto format, which would (in my current knowledge) not be used for sending emails.
- One thing I found that may be missing in the code base was a standardized way of handling errors in C++ code. Since exceptions are disabled I think that the functional way is the way to go. After some research I found the Expected monad / tagged union / sum type, which seemed to suit the problem particularly well.

In the long run, I hope we would move the entire code base to use `Expected` to indicate if a function might fail.

Of course every choice made here is to be considered as a proposition for doing things / RFC, critics wholeheartedly accepted.

Reviewers: cmollekopf

Tags: #kube

Maniphest Tasks: T6994, T8147, T6995

Differential Revision: https://phabricator.kde.org/D11158

1 files changed, 1 insertions, 1 deletions

diff --git a/framework/src/domain/mime/mailtemplates.h b/framework/src/domain/mime/mailtemplates.h
index 9447e169..154b76a2 100644
--- a/framework/src/domain/mime/mailtemplates.h
+++ b/framework/src/domain/mime/mailtemplates.h
@@ -38,5 +38,5 @@ namespace MailTemplates
     void forward(const KMime::Message::Ptr &origMsg, const std::function<void(const KMime::Message::Ptr &result)> &callback);
     QString plaintextContent(const KMime::Message::Ptr &origMsg);
     QString body(const KMime::Message::Ptr &msg, bool &isHtml);
-    KMime::Message::Ptr createMessage(KMime::Message::Ptr existingMessage, const QStringList &to, const QStringList &cc, const QStringList &bcc, const KMime::Types::Mailbox &from, const QString &subject, const QString &body, bool htmlBody, const QList<Attachment> &attachments, const std::vector<GpgME::Key> &signingKeys = {}, const std::vector<GpgME::Key> &encryptionKeys = {});
+    KMime::Message::Ptr createMessage(KMime::Message::Ptr existingMessage, const QStringList &to, const QStringList &cc, const QStringList &bcc, const KMime::Types::Mailbox &from, const QString &subject, const QString &body, bool htmlBody, const QList<Attachment> &attachments, const std::vector<GpgME::Key> &signingKeys = {}, const std::vector<GpgME::Key> &encryptionKeys = {}, const GpgME::Key &attachedKey = {});
 };