This commit adds functionality for ox-icalendar to set X-PUBLISHED-TTL
in the exported ICS, which advises a subscriber to the exported ICS
file to reload after the given time interval.
* lisp/ox-icalendar.el (org-icalendar-ttl): New option to set
X-PUBLISHED-TTL in exported ICS
(icalendar): Add ICAL-TTL export keyword
(org-icalendar--vcalendar): Add argument for TTL
(org-icalendar-template, org-icalendar-export-current-agenda,
org-icalendar--combine-files): Pass TTL to `org-icalendar--vcalendar'
Co-authored-by: Ihor Radchenko <yantar92@posteo.net>
Co-authored-by: Jack Kamm <jackkamm@gmail.com>
da2b61b09e