Notify deletion in two steps.

Notify clients before we actually remove the incidence in notifyIncidenceAboutToBeDeleted.
And after the deleted with notifyIncidenceDeleted, if the incided was removed from the calendar.

API addition:

calendarIncidenceDeleted now have a pointer to the calendar.
It needs to provide a pointer to the calendar, because otherwise
that's impossible to find out (the incidence is already removed,
at that point).

Differential revision: https://phabricator.kde.org/D589