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, 0 deletions

diff --git a/framework/qml/Icons.qml b/framework/qml/Icons.qml
index 2afe840e..4dfae3d7 100644
--- a/framework/qml/Icons.qml
+++ b/framework/qml/Icons.qml
@@ -63,6 +63,7 @@ Item {
     property string secure: "document-encrypt"
     property string insecure: "document-decrypt"
     property string signed: "document-sign"
+    property string key_import_inverted: "view-certificate-import-inverted"
 
     property string addNew: "list-add"
     property string remove: "kube-list-remove-inverted"