Eliminate the warning about serialization in non-public API of Swing

As part of the serialization cleanup, I would like to remove the serialization warning below from the non-public API(mostly motif and windows L&Fs). Later I will remove it from the public API and move this warning to the "javax.swing" package spec.

 * <strong>Warning:</strong>
 * Serialized objects of this class will not be compatible with
 * future Swing releases. The current serialization support is appropriate
 * for short term storage or RMI between applications running the same
 * version of Swing. A future release of Swing will provide support for
 * long term persistence.