Fix inline responses by making them sync

* src/mail.cpp (Mail::is_inline_response),
(Mail::check_inline_response): New.
(do_crypt): Work sync for inline response.
* src/mailitem-events.cpp: Work sync for inline response.

Its nice how we can switch between sync and async in the new
architecture. Hopefully this does not mean double bugs ;-)

Working sync for inline response is necessary because we
can't trigger a send event programatically for inline responses.