Show body of MIME messages
* src/mailitem-events.cpp (MailItemEvents::handle_after_write), (MailItemEvents::handle_before_read): New. Moved out of Invoke. (MailItemEvents::handle_read): New. Set the body to the decrypted / verified content.
This should not yet be used. The body modification in the OOM
causes the plaintext to be persistently stored.