Add object counting to cdef_class. * libinterp/octave-value/ov-classdef.h (cdef_class::cdef_class_rep::object_count): New member. (cdef_class::cdef_class_rep::cdef_class_rep): Initialize it. (cdef_class::cdef_class_rep::register_object, cdef_class::cdef_class_rep::unregister_object): New methods. (cdef_class::register_object, cdef_class::unregister_object): Likewise. (cdef_object_base::set_class, cdef_object_base::register_object, cdef_object_base::unregister_object): Use them. * libinterp/octave_value/ov-classdef.cc (cdef_class::cdef_class_rep::cdef_class_rep (std::list<cdef_class>)): Initialize object_count.