Process HDN_ITEMCHANGING notifications only when column is being resized

When column resizing is finished, after HDN_ENDTRACK notification there is
also sent one (and last) HDN_ITEMCHANGING notification. We have to skip it
to prevent from sending EVT_HEADER_RESIZING after EVT_HEADER_END_RESIZE
because EVT_HEADER_END_RESIZE should be really the last one event
in the sequence of resizing events (like it's assumed in wxGrid).

Closes #16390.

include/wx/msw/headerctrl.h

@@ -139,6 +139,9 @@ private:
     // actual column we are dragging or -1 if not dragging anything
     int m_colBeingDragged;
 
+    // a column is currently being resized
+    bool m_isColBeingResized;
+
     // the custom draw helper: initially NULL, created on demand, use
     // GetCustomDraw() to do it
     wxMSWHeaderCtrlCustomDraw *m_customDraw;