itip-formatter: Send meeting invitations as multipart/alternative
Evolution sends invititions as Content-Type: text/calendar; METHOD="REQUEST".
RFC 6047 suggests sending iCalendar invitations as multipart/alternative containing text/calendar and text/plain MIME parts, so that MUAs not understanding text/calendar can still show to the user the content. This is usefull e.g. when sendung invitations over mailing lists: some users read about an event and click on “Include in my calendar”, other users don’t click and only read the iCalendar files, and the third group of recipients only reads the text part (or HTML part, if there is such an alternative form).