The correct sequence of actions should be remove(old) → insert(new), not
insert(new) → remove(old). We can implement a simple delegate insertion
functions to insert the new child between the previous and next siblings
of the old child.
While we're at it, let's also add a unit test for replace_child().
Verify that insertion and removal maintain a stable graph, with pointers
to the various children. This should help out tracking regressions in
the scene graph API.
