function PrefixedPreferencesSystem(base, prefix) {
        this.base = base;
        this.prefix = prefix;
        this._listenerInstalled = false;
    }
Provides a subset of the PreferencesSystem functionality with preference
access always occurring with the given prefix.

@constructor
@param {PreferencesSystem} base The real PreferencesSystem that is backing this one
@param {string} prefix Prefix that is used for preferences lookup. Any separator characters should already be added.