Importing events from an ICS should preserve event ID

[tsheinen]

Per RFC 5545 every ical VEvent object is required to have a string UID property which is used for duplicate detection. Ideally this should be copied into the "id" property of the event that CreateEventFromVOBJ returns so that importing duplicate events will not create duplicate events on google calendar. I believe this is probably the best solution to #492 because google calendar will handle deduplication on its own if the IDs are the same.