IDLE: Factor ConfigChanges class from configdialog, put in config; test. *
In config, put dump test code in a function; run it and unittest in   'if
__name__ == '__main__'. * Add class config.ConfigChanges based on
changes_class_v4.py on bpo issue. * Add class test_config.ChangesTest,
partly using configdialog_tests_v1.py. * Revise configdialog to use
ConfigChanges; see tracker msg297804. * Revise test_configdialog to match
configdialog changes. * Remove configdialog functions unused or moved to
ConfigChanges. Cheryl Sabella contributed parts of the patch.
