ofEventListener: new method isSubscribed()

It would be useful to know if an ofEventListener is currently subscribed to something.

Currently we can explicitly unsubscribe(), but there it would be nice to check something like:

class ofEventListener{
public:
	ofEventListener(){}
	ofEventListener(std::unique_ptr<of::priv::AbstractEventToken> && token)
	:token(std::move(token)){}
	void unsubscribe(){
		token.reset();
	}

        bool isSubscribed() const
        {
               return token != nullptr;
        }
private:
	std::unique_ptr<of::priv::AbstractEventToken> token;
};

Start by locating the ofEventListener class and its existing unsubscribe() implementation. Add coverage for the subscribed and unsubscribed states, then verify the event-listener tests pass and that the new query reports whether a listener currently holds a subscription.