Creates a new renderable for the given renderable-id.
function _createRenderable (id, data) {
        var option = this.options.createRenderables[id];
        if (option instanceof Function) {
            return option.call(this, id, data);
        }
        else if (!option) {
            return undefined;
        }
        if ((data !== undefined) && (data instanceof Object)) {
            return data;
        }
        var surface = new Surface({
            classes: this.classes,
            content: data ? ('<div>' + data + '</div>') : undefined
        });
        surface.addClass(id);
        if (id === 'item') {
            if (this.options.tabBarLayout && this.options.tabBarLayout.itemSize && (this.options.tabBarLayout.itemSize === true)) {
                surface.setSize(this.layout.getDirection() ? [undefined, true] : [true, undefined]);
            }
        }
        return surface;
    }