TADS 3 Library - settings file management

This is a framework that the library uses to keep track of certain preference settings - things like the NOTIFY, FOOTNOTES, and EXITS settings.

The point of this framework is "global" settings - settings that apply not just to a particular game, but to all games that have a particular feature. Things like NOTIFY, FOOTNOTES, and some other such features are part of the standard library, so they tend to be available in most games. Furthermore, they tend to work more or less the same way in most games. As a result, a given player will probably prefer to set the options a particular way for most or all games. If a player doesn't like score notification, she'll probably dislike it across the board, not just in certain games.

This module provides the internal, programmatic core for managing global preferences. There's no UI in this part of the implementation; the adv3 library layers the UI on top via the settingsUI object, but other alternative UIs could be built using the API provided here.

The framework is extensible - there's an easy, structured way for library extensions and games to add their own configuration variables that will be automatically managed by the framework. All you have to do to create a new configuration variable is to create a SettingsItem object to represent it. Once you've created the object, the library will automatically find it and manage it for you.

This module is designed to be separable from the adv3 library, so that alternative libraries or stand-alone (non-library-based) games can reuse it. This file has no dependencies on anything in adv3 (at least, it shouldn't).