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.