Add wxDateTime::GetWeekBasedYear().

It was just added as a private function to implement %V format specifier
support, just extract and document it as it could possibly be useful in its
own right.

See #11857.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76989 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/datetime.h

@@ -760,6 +760,9 @@ public:
         // invalid)
     wxDateTime_t GetWeekOfYear(WeekFlags flags = Monday_First,
                                const TimeZone& tz = Local) const;
+        // get the year to which the number returned from GetWeekOfYear()
+        // belongs
+    int GetWeekBasedYear(const TimeZone& tz = Local) const;
         // get the week number since the month start (1..5, 0 if date is
         // invalid)
     wxDateTime_t GetWeekOfMonth(WeekFlags flags = Monday_First,