convert_child_iter_to_iter
Description:
Sets filter_iter to point to the row in this that corresponds to the row pointed at by 
        child_iter.
 If filter_iter was not set, false is returned.
Parameters:
| this | A `GtkTreeModelFilter` | 
| filter_iter | An uninitialized `GtkTreeIter` | 
| child_iter | A valid `GtkTreeIter` pointing to a row on the child model. | 
Returns:
| true, if  |