Move Print object storage to C++. (along with its subobjects)
Call Print object's set_total_ functions correctly.
Add ::Ref classes for every missing C++ class (going by REGISTER_CLASS calls).
Return NULL values from XS as undef.
Try to fix _copies_shift scaling.
Remove some more Clone typemaps and add some whitespace.
Make read-only constructor parameters - private.
Make returned PrintObject from Layer accessor a Ref.
Set ISA manually for Slic3r::Layer.