Show recurring to-dos on DTDUE in the month view, like plain to-dos
The month view shows non-recurring multi-day to-dos on their due date,
but shows recurring to-dos on their start date. The cause is that
startDate()
ultimately calls Todo::dateTime(Incidence::RoleDisplayStart)
,
which returns dtDue()
instead of dtStart()
, so the offset to the
start date is miscalculated.