Automatically determine the first weekday for a calendar control

If neither wxCAL_SUNDAY_FIRST or wxCAL_MONDAY_FIRST was given, use
wxDateTime::GetFirstWeekDay() to automatically determine the preferred
day.

This changes the earlier default behaviour, which was to use Sunday
if not otherwise specified. However, the wxGTK native calendar control
ignored this behaviour anyway.

include/wx/generic/calctrlg.h

@@ -202,13 +202,13 @@ private:
     // get the first/last days of the week corresponding to the current style
     wxDateTime::WeekDay GetWeekStart() const
     {
-        return HasFlag(wxCAL_MONDAY_FIRST) ? wxDateTime::Mon
+        return WeekStartsOnMonday() ? wxDateTime::Mon
                                            : wxDateTime::Sun;
     }
 
     wxDateTime::WeekDay GetWeekEnd() const
     {
-        return HasFlag(wxCAL_MONDAY_FIRST) ? wxDateTime::Sun
+        return WeekStartsOnMonday() ? wxDateTime::Sun
                                            : wxDateTime::Sat;
     }