Add first (dummy) version of C++ MailParser

* src/mail.cpp (get_cipherstream): Use second
attachment for now.
(use_body): Small helper to detect if the body
should be used.
(Mail::decrypt_mime): Removed.
(Mail::decrypt): Removed.
(add_attachments): New helper to create OOM
attachments from Attachment Objects.
(Mail::decrypt_verify): Use MailParser.
(Mail::get_subject): New helper.
* src/mailparser.cpp, src/mailparser.h: New.

Instead of refactoring the mimeparser to work without MAPI
structures I'm not incrementally factoring things out from
there into a new MailParser class which will replace both
message.cpp and mimeparser.c and should work without writing
directly to MAPI so that in the future the MailParser can
be moved into a different thread and we avoid chaning
underlying MAPI Stores without going through oom.