Make Print subobject constructors and destructors private/protected and virtual.
Simplify Print subobject constructors and call them only through add_/set_new_ methods.
Use new layers array accessors from Perl code.
Add PrintObject->layers and ->support_layers methods to XSP.
Make PrintObject::add_[support_]layer create the objects themselves.
Wrap PrintObject's layer methods in XSP.