Add GetUpdatedAscendingSortIndicator() helper function

It seems like this function will need to be used in every implementation
of EVT_LIST_COL_CLICK handler when using sorting, so it makes sense to
provide it in the library itself.

interface/wx/listctrl.h

@@ -1460,6 +1460,31 @@ public:
     */
     int GetSortIndicator() const;
 
+    /**
+        Returns the new value to use for sort indicator after clicking a
+        column.
+
+        This helper function can be useful in the EVT_LIST_COL_CLICK handler
+        when it updates the sort indicator after the user clicked on a column.
+
+        For example:
+        @code
+            void MyListCtrl::OnColClick(wxListEvent& event)
+            {
+                int col = event.GetColumn();
+                if ( col == -1 )
+                    return; // clicked outside any column.
+
+                const bool ascending = GetUpdatedAscendingSortIndicator(col);
+                SortItems(MyCompareFunction, ascending);
+                ShowSortIndicator(col, ascending);
+            }
+        @endcode
+
+        @since 3.1.6
+    */
+    bool GetUpdatedAscendingSortIndicator(int col) const;
+
     /**
         Returns @true if the sort indicator direction is ascending,
         @false when the direction is descending.