notmuch-show.el: import calendar data with public function after CR removal

notmuch-show.el: import calendar data with public function after CR removal

notmuch-get-bodypart-content provides raw data to its caller so
that it can be stored verbatim whenever needed. icalendar functions
expect Emacs to do EOL conversion for the data given to these. Therefore
it the CRLF -> LF conversion is now done explicitly.

The calls to private functions icalendar--convert-ical-to-diary and
icalendar--read-element are replaced with call to public function
icalendar-import-buffer.