Store the org-generated UID, to allow for multiply occurring events

When a given headline contains several timestamps, the icalendar exporter will generate one event per timestamp, with UIDs prefixed TS1-, TS2- and so on. Rather than trimming it, keep the different versions and only trim the prefix when calling org-id-*. This feels simpler, and besides it allows for the new feature of the exporter to be exploited cleanly.