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.

include/wx/headerctrl.h

@@ -159,6 +159,12 @@ public:
     // compute column title width
     int GetColumnTitleWidth(const wxHeaderColumn& col);
 
+    // compute column title width for the column with the given index
+    int GetColumnTitleWidth(unsigned int idx)
+    {
+        return GetColumnTitleWidth(GetColumn(idx));
+    }
+
     // implementation only from now on
     // -------------------------------