Add interfaces to custom types before class_init.

* glib/glibmm/class.[h|cc]: Add a clone_custom_type() overload that takes
a vector of pointers to Interface_Class instances, so we can call their
add_interface() functions on the GType just after registering it, but before
instantiating the first GObject.
custom_class_init_function(): Override properties of implemented interfaces
that have not been overridden in a base class.
* glib/glibmm/interface.cc:
Interface::Interface(const Interface_Class& interface_class):
If the GObject has not been instantiated yet, then add interface_class to
the Class::custom_interface_classes vector.
* glib/glibmm/objectbase.[h|cc]: Add extra_object_base_data and
extra_object_base_data_mutex.
* glib/glibmm/objectbase.cc: ~ObjectBase(): Erase 'this' from
extra_object_base_data.
* glib/glibmm/object.cc: Default constructor and
Object::Object(const Glib::ConstructParams& construct_params): Pass the list
of Interface_Class pointers to the new Class::clone_custom_type() method
overload.  Bug #697229.