public function renderAll()
    {
        $widgets = [];

        foreach ($this->getFilters() as $field => $options) {
            $w = $this->filterWidget($field, $options, false);
            if ($w) {
                $widgets = array_merge($widgets, $w);
            }
        }
        // FIXME allow column-layout to be configured with templates.
        $ret = '<div class="row">';
        foreach ($widgets as $i => $widget) {
            $ret .= '<div class="col-md-6">';
            $ret .= $widget;
            $ret .= '</div>';
            if (($i + 1) % 2 === 0) {
                $ret .= '</div><div class="row">';
            }
        }
        $ret .= '</div>';

        return $ret;
    }