Deprecate (and eventually remove) the WTKListenerList class, which is currently empty

The WTKListenerList class was originally added as a subclass of ListenerList, but with thread-safety checks in it (basically checking that all the operations were performed on the Event Dispatch Thread (EDT) or "main" during startup). All these checks were removed some time ago, so that component hierarchies could be built "offline", that is in a background thread, either by BXMLSerializer, or programmatically. This left WTKListenerList as an empty class, but with comments to the effect that it was used to provide thread safety checks.... This is confusing, and unnecessary. So, I propose to deprecate the class in version 2.1, replacing all the internal references with just "ListenerList", and so the class can be removed at some point after that.

Note: deprecating it in the 2.0.x branch is not really an option, since it means the build gets 100s of warnings about the deprecation, which is not good for a release branch.