Fetches image and their ids from the client.

def refresh(self):
        """
        Fetches image and their ids from the client.
        """
        if not self._client:
            return
        current_images = self._client.images()
        self.clear()
        self._update(current_images)
        for image in current_images:
            tags = image.get('RepoTags')
            if tags:
                self.update({tag: image['Id'] for tag in tags})