Listener

This class is responsible of doing the connection of a Subscriber to a Publisher for an Event of a given type. Listener have a unique ID, which is used to retrieve it from a Subscriber, in order to disconnect it or add EventFilters if necessary.
Another derived class ListenerForEvent<EVENT> can also be used. This class specialize the Listener class for a given event template parameter EVENT. The pure virtual method process_event(Event*) is replaced by the specialized one process_event(EVENT*), where EVENT is the template parameter.