(Backends) Replace 'printInstances' and 'printEntities' method by a more
generic 'printChildren' that calls the right printing function
according to the type of each child.
(Core) Add incomingPortMap and outgoingPortMap to Actor.
(Core) Remove deprecated getId() method from Instance.
(C) Remove all uses of 'instance' attribute in the printer to prepare
the code generation from actors.