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, 10 insertions, 3 deletions

diff --git a/framework/src/domain/composercontroller.cpp b/framework/src/domain/composercontroller.cpp
index 2286a71b..a71e66f9 100644
--- a/framework/src/domain/composercontroller.cpp
+++ b/framework/src/domain/composercontroller.cpp
@@ -135,7 +135,7 @@ public:
         SinkLog() << "Searching key for: " << mb.address();
         asyncRun<std::vector<GpgME::Key>>(this,
             [mb] {
-                return MailCrypto::findKeys(QStringList{} << mb.address(), false, false, MailCrypto::OPENPGP);
+                return MailCrypto::findKeys(QStringList{} << mb.address(), false, false);
             },
             [this, addressee, id](const std::vector<GpgME::Key> &keys) {
                 if (!keys.empty()) {
@@ -463,18 +463,25 @@ KMime::Message::Ptr ComposerController::assembleMessage()
         };
     });
 
+    GpgME::Key attachedKey;
     std::vector<GpgME::Key> signingKeys;
     if (getSign()) {
         signingKeys = getPersonalKeys().value<std::vector<GpgME::Key>>();
+        Q_ASSERT(!signingKeys.empty());
+        attachedKey = signingKeys[0];
     }
     std::vector<GpgME::Key> encryptionKeys;
     if (getEncrypt()) {
         //Encrypt to self so we can read the sent message
-        encryptionKeys += getPersonalKeys().value<std::vector<GpgME::Key>>();
+        auto personalKeys = getPersonalKeys().value<std::vector<GpgME::Key>>();
+
+        attachedKey = personalKeys[0];
+
+        encryptionKeys += personalKeys;
         encryptionKeys += getRecipientKeys();
     }
 
-    return MailTemplates::createMessage(mExistingMessage, toAddresses, ccAddresses, bccAddresses, getIdentity(), getSubject(), getBody(), getHtmlBody(), attachments, signingKeys, encryptionKeys);
+    return MailTemplates::createMessage(mExistingMessage, toAddresses, ccAddresses, bccAddresses, getIdentity(), getSubject(), getBody(), getHtmlBody(), attachments, signingKeys, encryptionKeys, attachedKey);
 }
 
 void ComposerController::send()