Well, the sidebar isn't a widget by definition, but you can add a title to it anyway. You can do that in a few ways but I thing the most clean way, considering you're using Q2A 1.7, is to process it in the initialize() function this way:
public function initialize() {
$this->content['sidebar'] = isset($this->content['sidebar']) ? $this->content['sidebar'] : '';
$this->content['sidebar'] = '<h1>Title!</h1>' . $this->content['sidebar'];
parent::initialize();
}