Implement some embryonic handle-classdef semantic. * libinterp/octave-value/ov-classdef.h (cdef_object_rep::subsasgn, handle_cdef_object::subsasgn, octave_classdef::subsasgn): New method declarations. (handle_cdef_object::~handle_cdef_object): New destructor declaration. (cdef_class::cdef_class_rep::handle_class): New boolean field. (cdef_class::cdef_class_rep::cdef_class_rep): Initialize it. (cdef_class::cdef_class_rep::mark_as_handle_class, cdef_class::cdef_class_rep::is_handle_class, cdef_class::mark_as_handle_class, cdef_class::is_handle_class): Manipulate it. (cdef_class::cdef_class_rep::get_name): New method. (cdef_class::get_name): Use it. (cdef_class::cdef_class_rep::initialize_object, cdef_class::cdef_class_rep::subsref_meta, cdef_class::cdef_class_rep::run_constructor, cdef_class::cdef_class_rep::construct, cdef_class:initialize_object, cdef_class::subsref_meta, cdef_class::run_constructor, cdef_class::construct): New methods. (cdef_class::cdef_class_rep::find_method, cdef_class::find_method): Add "local" argument. (cdef_class::cdef_class_rep::find_names, cdef_class::find_names): Change signature to use std::set and a boolean flag. (cdef_class::cdef_class_rep::find_methods, cdef_class::find_methods): Change signature to use a boolean flag as second argument. (cdef_class::make_meta_class): New static method. (cdef_class::get_method_function, cdef_class::get_constructor_function): New methods. (cdef_property::cdef_property_rep::set_value, cdef_property::set_value): Make cdef_object argument non const. (cdef_property::cder_property_rep::is_relative_set): New method. (cdef_property::get_get_access, cdef_property::get_set_access): Delete methods. (cdef_property::check_get_access, cdef_property::check_set_access): Remove string argument. (cdef_method::get_access): Delete method. (cdef_method::check_access): Remove string argument. * libinterp/octave-value/ov-classdef.cc (gripe_method_access, gripe_property_access): Support access specified as cell array of classes. (make_function_of_class): New static function(s). (check_access (std::string, std::string), superclass_access): Remove static functions. (lookup_class): Use symbol_table when class hasn't been loaded yet. (lookup_classes): Returns std::list<cdef_class> instead of Cell. (class_get_superclasses, class_get_inferiorclasses): Use it. (to_ov (const std::list<cdef_class>&)): New static function. (get_class_context, check_access (const cdef_class&, const octave_value&)): Likewise. (handle_cdef_object::subsref): Use new signature of access check methods. (property_get_defaultvalue): New built-in property accessor. (make_class): Change signature to support multiple inheritance. Set "Sealed" to false by default. Determine value for HandleCompatible property and handle-class representation. (make_property): Take cdef_class as first argument. Add DefaultValue and HasDefault properties. Call make_function_of_class for property accessors. (make_attribute): Take cdef_class as first argument. (make_method): Likewise. Call make_function_of_class. (make_method (octave_builtin::fcn)): Do no construct a function handle object. (octave_classdef::subsasgn, handle_cdef_object::subsasgn): New method. (class octave_classdef_proxy): New class. (cdef_class::get_method_function): Use it, new method. (handle_cdef_object::~handle_cdef_object): New destructor. (cdef_class_rep::find_method): New boolean "local" argument. When true, only look into the current class, not in superclasses. (cdef_class_rep::find_methods): New signature. (cdef_class_rep::get_methods): Use it. (cdef_class_rep::find_properties): New signature. (cdef_class_rep::get_properties): Use it. (cdef_class_rep::find_names): New signature. (cdef_class_rep::get_names): Use it. (cdef_class_rep::subsref_meta, cdef_class_rep::initialize_object, cdef_class_rep::run_constructor, cdef_class_rep::construct): New methods. (compute_attribute_value, attribute_to_string): New static functions. (cdef_class::make_meta_class): Change signature, non const argument. Implement it. (cdef_property_rep::get_value): Do not check access here. (cdef_property_rep::set_value, cdef_property_rep::is_recursive_set): New method. (cdef_property_rep::check_get_access, cdef_property_rep::check_set_access, cdef_method_rep::check_access): Use static check_access utility function. (install_classdef): Adapt to change of signature of make_class. Mark meta classes as sealed. Add HandleCompatible property to meta.class. Add DefaultValue and HasDefault properties to meta.property. * libinterp/parse-tree/pt-classdef.h (tree_classdef::make_meta_class): Change return type to (octave_function *). * libinterp/parse-tree/pt-classdef.cc (tree_classdef::make_meta_class): Likewise. Call cdef_class::get_constructor_function. * libinterp/parse-tree/oct-parse.yy (parse_fcn_file): Adapt to new signature of tree_classdef::make_meta_class. * libinterp/octave-value/ov-fcn.h (octave_function::is_classdef_constructor): New virtual method. * libinterp/octave-value/ov-usr-fcn.h (octave_user_function::class_ctor_type): New private enum. (octave_user_function::mark_as_class_constructor, octave_user_function::is_class_constructor): Use it. (octave_user_function::mark_as_classdef_constructor, octave_user_function::is_classdef_constructor): New methods. (octave_user_function::class_constructor): Turn into class_ctor_type. * libinterp/octave-value/of-usr-fcn.cc (octave_user_function::octave_user_function): Initialize class_constructor. (octave_user_function::do_multi_index_op): When function is a classdef constructor, extract the first argument and use it to populate the first output argument.