Added DebugMenu listener

Listener that handles both direct observation of notifications or can register an observer and selector via the DebugMenu interface
Rather than waiting until the keys are set to subscribe, it might be better to just subscribe to the notification in `init`. In fact, I don't think it's necessary to set the keys here at all. Instead, just expose a readonly set of all the settings keys on the settings interface. Then, in `RZDebugMenu`, before you register an observer with this class, first check that the key is included in the settings keys and if not, log the warning there before ever calling into this class. This will help keep this class's responsibilities simple and bubble up some of that logic.