Feature void event

Adds the possibility for an ofEvent<void> that can be listened to, from a method without arguments.

Don't merge yet, this modifies some Poco files i've sent a message in the poco forums to check if they have any interest in this change. Poco has a Poco::Void class that can be used to simulate something like this but the listener method has to receive a Poco::Void and on notify you need to create the variable before calling like:

    Poco::Void v;
    ofNotifyEvent(event,v);

which seems way less useful than a true void event but probably a reason not to accept my patch