Add DeletionList class
load-3ds.cc: Disable weird material frobbing

It doesn't really serve any purpose now that meshes can only hold a
single material, and it will interfere with future changes.
Be more careful about disowning pointers in SWIG
Simplify SWIG scene __str__ method

The info previously included wasn't very useful anyway.
Remove Scene::num_surfaces method
Tweak Space method comments