Add revert to mail class and use it instead wipe

* src/mail.cpp (Mail::revert_all_mails, Mail::revert): New.
* src/mail.h: Update accordingly.
* src/mailitem-events.cpp (MailitemEvents::Invoke): Revert instead
  of wipe.

The wipe code remains intact for now. If we find out that reverting
is the wrong approach we can fall back to wipining again by changing
the call in mailitem-events.

For now most messages will be wiped anyway as revert is only
supported for PGPMESSAGE messages.