Timestamps in properties not exported by icalendar exporter

[David Engster <deng@randomsample.de>]

I have the problem that a certain kind Org entries is not exported by
the icalendar exporter, namely those created by the gnus-icalendar
package.

This package creates Org entries from calendar invites in the following
way (I've omitted some of the properties, but you get the idea);

** Some appointment (location)
   :PROPERTIES:
   :ICAL_EVENT: t
   :ID:       040000008200E00074C5B7101A82E00800
   :DT:       <2013-12-09 Mon 13:00-13:30>
   :END:

   Body text


These entries show up in the agenda just fine, but the icalendar
exporter does not export it because the timestamp is in the properties
(the gnus-icalendar package puts it there so that you can easily change
it if the organizer decides to change the event and sends a change
request).

The exporter could of course simply take the first timestamp it finds in
the properties, but it could be the wrong one; for instance, there could
be a LOGBOOK timestamp before the one in DT. Hence I think it would be
nice if you could tell the exporter which property to use as timestamp
during export.

-David