ExternalSettings manages settings kept externally, e.g. files.
Objects can register themselves as clients to be notified at startup time to read their settings.

Eventually all the preferences should be managed through this mechanism.

	class initialization
	initialize shutDown startUp

	private
	registeredClients

	accessing	top
	assuredPreferenceDirectory
	preferenceDirectory
	preferenceDirectoryName
	registerClient: Register anObject as a settings client to be notified on startup.

	class initialization	top
	initialize ExternalSettings initialize
	shutDown Look for external defs and load them.
	startUp Look for external defs and load them.

	private	top
	registeredClients