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, 0 deletions

diff --git a/framework/qml/AttachmentDelegate.qml b/framework/qml/AttachmentDelegate.qml
index 3c308e65..4469cbdd 100644
--- a/framework/qml/AttachmentDelegate.qml
+++ b/framework/qml/AttachmentDelegate.qml
@@ -23,10 +23,12 @@ Item {
     id: root
 
     property string name
+    property string type
     property string icon
     property alias actionIcon: actionButton.iconName
     signal clicked;
     signal execute;
+    signal publicKeyImport;
 
     width: content.width + Kube.Units.smallSpacing * 1.5
     height: content.height + Kube.Units.smallSpacing
@@ -70,6 +72,14 @@ Item {
             color: Kube.Colors.backgroundColor
         }
         Kube.IconButton {
+            visible: root.type == "application/pgp-keys"
+            iconName: Kube.Icons.key_import_inverted
+            height: Kube.Units.gridUnit
+            width: height
+            onClicked: root.publicKeyImport()
+            padding: 0
+        }
+        Kube.IconButton {
             id: actionButton
             height: Kube.Units.gridUnit
             width: height