No crash on trying to duplicate read-only events
No crash on trying to duplicate read-only events

Obviously this solution is stupid because you might want to duplicate
an event from a read-only calendar, but it's too much work to fix this
properly because the whole EventColumn set up is getting overhauled in
branch `feature_daywalker` anyway.

hobarrera/khal,sdx23/khal,pdav/khal,pimutils/khal,geier/khal