Make QCalendarWidget use its locale's firstDayOfWeek.

Make QCalendarWidget use its locale's firstDayOfWeek.

QCalendarWidget currently uses Qt::Sunday as the default first day
of the week. It has been suggested that a better user experience
would see the calendar's locale be used instead.

Task-number: QTBUG-19811
Change-Id: I4441bf9ffd52213ef622a4a7f498530b7cc9e110
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Denis Dzyubenko <denis.dzyubenko@nokia.com>