Enable customization/optimization of ListenerManager by making it possible to override smoothly via subclassing

[dbotwinick]

Changed instantiation of ListenerManager to use a protected method to facilitate customization via subclassing. For my use case, I wanted to simplify the ListenerManager since I only have the use case of a single, fixed listener. I felt this change would essentially have zero impact on others but would enable my use-case without much complexity.