In plug_in_compat.pdb, don't add child nodes to nodes containing an
op, since this turns them into graphs and discards the op.
Instead, add a new wrap_in_graph() helper function, which takes a
node op and wraps it in a simple "input -> op -> output" graph.
Use the graph as the container for child nodes, and as the node
passed to gimp_drawable_apply_operation().  (This is similar to
what we used to do before commit
afdd5731, except that we now pass
the parent node to gimp_drawable_apply_operation(), instead of the
op node).