fix MDN, iCalender Sender related issues, improve Sender: handling

See issue #79 (closed) for a detailed description. This patch

• fixes checking if a MDN request is suspicious according to RFC 8098,
• fixes sending an iCalendar reply to the ORGANIZER as requested by RFC 5545,
• removes the misleading _LibBalsaMessage::sender item,
• adds the _LibBalsaMessageHeaders::sender item and fills it appropriately,
• adds the Sender: header to the translatable headers for display and printing.

Details:

• libbalsa/mailbox_imap.c: ignore the IMAP envelope reply-to and sender items;
• libbalsa/message.[ch]: remove _LibBalsaMessage::sender, libbalsa_message_[gs]et_sender(); add _LibBalsaMessageHeaders::sender and set it from gmime; do not treat Sender: as misc header
• src/balsa-message.c: display all From: addresses in the message structure view; fix dealing with MDN requests according to RFC 8098
• src/balsa-mime-widget-vcalendar.c: send iCalendar reply according to RFC 5545
• src/balsa-mime-widget-message.c, src/balsa-print-object-header.c: make the Sender: header label translatable

Signed-off-by: Albrecht Dreß albrecht.dress@netcologne.de

Closes #79 (closed)