Add send_seen as member variable to Mail object

* src/mail.cpp, src/mail.h (Mail::needs_encrypt),
(Mail::set_needs_encrypt): New.
* src/mailitem-events.cpp (EVENT_SINK_INVOKE): Use new functions.

This puts the last state variable from the event handler into
the Mail object. It will be needed to Support Outlook 2007
where we have two event handlers.