Implement send again for crypto mails

* src/mail.cpp (Mail::set_is_send_again): New.
(Mail::parsing_done): Add handling for send again.
* src/mailitem-events.cpp: Detect a send again.

This detects the send again by the propchange of SendUsingAccount
and a "sent" value of false. Usually crypto mails have a sent
value of true.

In this case we remove our attachments (so that they are not
included in the sent again), set the draft info so that the
crypto state is nicely preselected and finally change the
type so that is_crypo_mail returns false for decrypted
send again mails.

• GnuPG-Bug-Id: T3885