Add an overload of ControlManager::removeListener() that takes an
additional ControlManager::ChangeType parameter, only the listener
for that change type is removed.  Used by VolumeFeedback to change
the listener for a volume change on a specified mixer, while still
maintaining the listener for a global master change.

Tidied up debug messages (log before the action to which it refers).
Make Listener a private struct (it doesn't need to be a public class).