[3.6] bpo-30779: IDLE -- Factor ConfigChanges class from configdialog, put in config...

[3.6] bpo-30779: IDLE -- Factor ConfigChanges class from configdialog, put in config; test. (GH-2612) (#2625)

* 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 based on configdialog_tests_v1.py on bpo issue.
* Revise configdialog to use ConfigChanges, mostly as specified in tracker msg297804.
* Revise test_configdialog to match configdialog changes.  All tests pass in both files.
* Remove configdialog functions unused or moved to ConfigChanges.
Cheryl Sabella contributed parts of the patch.
(cherry picked from commit 349abd9)