Event display can't show first event

[olantwin]

Because of several conditionals checking the truthiness of eventno, Event 0 can not be displayed.

The event number should probably be a required argument (as should be the other necessary arguments), and the conditionals can then be removed. Otherwise, the checks should test for eventno being None

[olantwin]

Maybe a good default behaviour would be to make event displays for all events (maybe with some maximum number in case of large trees).