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.

include/wx/datetime.h

@@ -392,6 +392,9 @@ public:
         // returns true if the given year is a leap year in the given calendar
     static bool IsLeapYear(int year = Inv_Year, Calendar cal = Gregorian);
 
+        // acquires the first day of week based on locale and/or OS settings
+    static bool GetFirstWeekDay(WeekDay *firstDay);
+
         // get the century (19 for 1999, 20 for 2000 and -5 for 492 BC)
     static int GetCentury(int year);