[FIX] get id from dataset to pass it as integer in place of string
[FIX]Bad view O2M with attribute delete
[FIX] missing class leading to broken list layout in o2m
[FIX] delete event in calendar, courtesy of Holger Brunn

Calendar has type mismatche issues where event ids are obtained as
strings even when they are numbers, this is (ish) why get_id_index was
added to Dataset.

When having an event_id in calendar, can't use event_id directly needs
to ensure we have valid index and use that index to get "correct (ly
typed)" id from dataset.