Wipe all decrypted mails on disconnect.

* src/mail.cpp (wipe_all_mails): New.
* src/gpgoladdin.cpp (~GpgOLAddin): Add more debug output.
  (OnDisconnection): Call wipe all mails.

This should cover the case that we decrypted a message and
are deactivated before the write / save event that removes
the plaintext in that message again.

nuPG-Bug-Id: 2138