Changing the type of reply from within the composer window

Gui: Change the reply mode from the compose window by means
of a dropdown menu from a tool button.

Switching to another (available) reply mode updates the recepients
list in the open compose window. If the "Hand Picked Recipients"
mode is picked, no action is triggered. Making manual changes to
recipients list reflects in the reply mode dropdown as a switch to
"Hand Picked Recipients" mode.

BUG: 321153
REVIEW: 118802
Gui: add icon for mail forward

icons/mail-forward is from oxygen's actions/32x32/mail-forward.png
licensed under LGPLv3

The name mail-forward is retained to allow picking up of the
relevant icon from a user's theme/icon-set other than oxygen.
Support for forwarding messages as attachments

Allow forwarding of a message as an attachment by means of menu actions.
Forwarding takes care of mangling the subject line and marking the
original message as "$Forwarded".

Thanks to paalsteek for his initial work on this.

BUG: 321378
REVIEW: 118606
Tests: Add test to ensure setting of $Forwarded flag after a message forward
Tests: add test rows for reply subject mangling to handle "Fwd"s
Tests: add test for subject mangling when forwarding message

REVIEW: 119615
Move processing of RFC2231-style string to a seperate method so it can be used in other places too
Remove usage of TreeItem from MsgPartNetworkReply
[gsoc] WIP
[gsoc][wip] first steps for new model
[gsoc][wip] new model can decrypt encrypted mails now
coding style: remove extra ;
[gsoc][wip] some progress towards displaying encrypted message parts
improved handling of children to avoid multiple decryption of a message
added and improved some roles for the message model
use nullptr instead of NULL or 0
debug tree view for decrypted messages and error handling for decryption added
added support for encrypted message/rfc822 parts and fixed downloading of encrypted multiparts
fixed cmake to build without qca
cmake improvements and cleanup