fix several bugs in index/position translation code between wx and MSW

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57343 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/headerctrl.h

@@ -90,6 +90,7 @@ private:
     int MSWFromNativeIdx(int item);
 
     // this is the same as above but for order, not index
+    int MSWToNativeOrder(int order);
     int MSWFromNativeOrder(int order);
 
     // get the event type corresponding to a click or double click event