Make MulticastParticipantRole constructor private.
Add Participant::get_core().
Implement ParticipantCore::create_multicast_participant_role().
Introduce Identity class (future replacement for Protocol::Participant).
Add copying constructor to Identity and remove copying assigment operator.
Add Identity to ParticipantCore and getter to participant.
Add ParticipantCore::drop_multicast_participant_role().
Introduce participant_row_box.glade, which will replace other row_boxes.
Introduce ParticipantCores class.
Add move constructor to Identity.
Add Participant::has_nay_roles() and move onstructor to ParticipantCore.