addEvent method
Implementation
void addEvent(CalendarEventData<T> event) {
assert(event.endDate.difference(event.date).inDays >= 0,
'The end date must be greater or equal to the start date');
// TODO: improve this...
if (_eventList.contains(event)) return;
if (event.isFullDayEvent) {
addFullDayEvent(event);
} else if (event.isRangingEvent) {
addRangingEvent(event);
} else {
addSingleDayEvent(event);
}
}