#6183: Fix conference serialization

sflphone-client-gnome/src/actions.c

sflphone-client-gnome/src/callable_obj.c

sflphone-client-gnome/src/callable_obj.h

sflphone-client-gnome/src/conference_obj.c

sflphone-client-gnome/src/conference_obj.h

sflphone-client-gnome/src/contacts/calltree.c

sflphone-client-gnome/src/contacts/conferencelist.c

sflphone-client-gnome/src/dbus/dbus.c

sflphone-common/src/history/historyitem.cpp

sflphone-common/src/history/historymanager.cpp

sflphone-common/test/history-sample.tpl

sflphone-common/test/historytest.cpp

sflphone-common/test/sflphoned-sample.yml