Return current object in SetContainer

Return $this is missing in the setContainer function.  It broke my code in the following example:
``` php
        $widgetType->setType($type)
            ->setCssDocument($this->getCSSDocument($location))
            ->setLocation($location)
            ->setEntity($widget)
            ->setWidgetManager($this)
            ->setContainer($this->getContainer())
            ->setSettings($this->getSettings($location, $id));
```