Support recurrence id to distinguish events with same uid

Quite untested at this point. Unit tests pass, though.

Recurrence id not yet exposed on QML interfaces. Event deletion also needs either to be moved to event/occurrence interfaces, or then Calendar API added with recurrence id parameter on remove() functions.

Disabled some event notebook change handling for now, not yet sure if that's the way notebook change should happen.

CalendarManager -> mEventOccurrencesForDates usage has been a bit broken already. Creating separate identifier in worker, but using event uid in NemoCalendarManager::updateAgendaModel(). Left untouched for now.