Reverted changes to MainMenu to simplify its methods.
Added Saving and Loading of Surveys and Tests

Added the ability to save and load Surveys and Tests from the MainMenu
using Serialization. This required having the Survey, Question,
Response, and InputOutput classes implement java.io.Serializable. All of
these classes and their subclasses also needed to a serialiVersionUID,
so the default was added to them all. Changes were also made to all
Response classes because Response was changed from an interface to an
abstract class.