* src/mail.cpp (Mail::Mail): Update init.
(Mail::get_sender): Sender is now a string.
(Mail::encrypt_sign): Update accordingly.
(Mail::update_sender): Changed to update_oom_data.
(Mail::update_oom_data): More generic. Caches BodyFormat,
HTMLBody and sender. Changed order for sender retrieval
when composing mails.
(Mail::update_sigstate): Update accordingly.
(Mail::is_html_alternative, Mail::get_cached_html_body): New.
* src/mail.h: Update accordingly.
* src/mailitem-events.cpp (EVENT_SINK_INVOKE): Update oom data
on send.
* src/mimemaker.cpp (create_mapi_attachment): Fix another use
after free.
(infer_content_type): is_mapibody now defines text/html or
text/plain. The naive content deduction is removed.
(add_body): Handler for the HTML / multipart body.
(do_mime_sign, mime_encrypt): Use add_body.
(mime_sign, mime_sign_encrypt): Pass on mail object.