get_selected_child
Description:
Gets the i-th selected Accessible child of an Selection.
  Note that selected_child_index refers to the index in the list  of 'selected'  children and generally differs from that used in 
       get_child_at_index or returned by  
      get_index_in_parent.  selected_child_index must lie
       between 0  and get_n_selected_children - 1, inclusive.
Parameters:
| this | a pointer to the Selection on which to operate. | 
| selected_child_index | a int indicating which of the selected children is specified. | 
Returns:
| a pointer to a selected Accessible child  object, specified by  |