Added an ExecutableActor class and modified PiMMVisitor and its subclasses wrt. it.
ExecutableActor is the abstract superclass of Actor, BroadcastActor, JoinActor, ForkActor and RoundBufferActor (basically, all AbstractActors except InterfaceActors).
It will allow us to apply different process indifferently to classical actors and specials actors.
Added Add and Create features for RoundBufferActors.
Updated GUI (PiMM editor and properties) and serialization wrt. new class ExecutableActor:
-A lot of features only owned by Actor are now shared by Actor and special actors through ExecutableActor (layout, name and ports management...)