Add wxHeaderCtrl::GetColumnTitleWidth() overload taking index

This makes it possible to get the appropriate column width from outside
the class, as GetColumn() itself is currently protected and so the
existing overload couldn't easily used.

interface/wx/headerctrl.h

@@ -404,6 +404,16 @@ public:
      */
     int GetColumnTitleWidth(const wxHeaderColumn& col);
 
+    /**
+        Returns width needed for the column with the given index.
+
+        This is just a convenient wrapper for the overload taking
+        wxHeaderColumn.
+
+        @since 3.1.3
+     */
+    int GetColumnTitleWidth(unsigned int idx);
+
 protected:
     /**
         Method to be implemented by the derived classes to return the