Implement new static function wxDateTime::GetFirstWeekDay()

This function tries to determine the preferred first day of week to use in
calendars. The procedure for obtaining this information is highly
platform-dependent, and is not possible on all platforms; in that case
Sunday is used as the fallback value.

Implementations are included for MSW, OSX, and Linux.

interface/wx/datetime.h

@@ -1483,6 +1483,18 @@ public:
     static bool IsDSTApplicable(int year = Inv_Year,
                                   Country country = Country_Default);
 
+    /**
+         Acquires the first weekday of a week based on locale and/or OS settings.
+         If the information was not available, returns @c Sun.
+         @param firstDay
+             The address of a WeekDay variable to which the first weekday will be
+             assigned to.
+	     @return If the first day could not be determined, returns false,
+		     and firstDay is set to a fallback value.
+         @since 3.1.1
+    */
+    static bool GetFirstWeekDay(WeekDay *firstDay);
+
     /**
         Returns @true if the @a year is a leap one in the specified calendar.
         This functions supports Gregorian and Julian calendars.