Converts child_path to a path relative to tree_model_sort. That is,
child_path points to a path in the child model. The returned path will
point to the same row in the sorted model. If child_path isn't a valid
path on the child model, then NULL is returned.
Converts child_path to a path relative to tree_model_sort. That is, child_path points to a path in the child model. The returned path will point to the same row in the sorted model. If child_path isn't a valid path on the child model, then NULL is returned.