Fix serialization of recurring to-do's start date
KCalendarCore uses Todo::Private::mDtRecurrence to hold the start date and time of the next uncompleted occurrance of a recurring Todo, and deserializes it from the X-KDE-LIBKCAL-DTRECURRENCE property of iCalendar files. However, ICalFormatImpl::writeTodo() serializes dtDue() into that property, not dtStart().
When KOrganizer creates a new recurring to-do, it serializes the to-do, then deserializes it to populate the To-Do List view, so the view shows an incorrect start date (and an incorrect due date, which is calculated from the start date).