Function to add to the list of available settings.
<p>Example usage:
<pre>
var settings = plugins.catalog.getObject('settings');
settings.addSetting({
name: 'tabsize', // For use in settings.get('X')
type: 'number',  // To allow value checking.
defaultValue: 4  // Default value for use when none is directly set
});
</pre>
@param {object} settingSpec Object containing name/type/defaultValue members.
function(settingSpec) {
        var setting = new Setting(settingSpec, this);
        this._settings[setting.name] = setting;
        this._settingNames.push(setting.name);
        this._settingNames.sort();
    }