alternatecalendar: port `QDate` to `QCalendar::YearMonthDay`

Not all dates from some alternate calendar systems can be accommodated
by QDate, for example, in Persian calendar, the second month has 31
days. QCalendar::YearMonthDay is a data structure to solve the problem.

`alternateDateReady` is not changed since more changes will depend on
some changes in KF6.

BUG: 463196
FIXED-IN: 5.27


(cherry picked from commit 64115353)