Add a scene_print function which prints a textual representation of the
current scene. This works by attaching a `debug_print` function to each
entity in the scene. This function is implemented for SACT2 and
GUIEngine entities. A default implementation will be used of none is
provided.
This can be accessed from the debugger via the `scene` command (although
the output is typically too much to view in a terminal; it's better to
run `scene_print` somewhere and redirect stdout to a file).