Generate zero-based orderings in reorder_paths
Closes #1546 (closed)
ROW_NUMBER() starts at 1, not 0.
Element.add_parent is working properly, since it uses COALESCE(MAX(ordering) + 1, 0): if there is no element in the parent, it defaults to zero. Only the orderings handled by reorder_paths on non-top-level elements are affected.