DX of responsive image is improved

It is now easy to output a responsive image from code

A picture tag can now be rendered by using

  $content['hero_image_responsive'] = array(
    '#type' => 'responsive_image',
    '#uri' => 'public://hero-image.png',
    '#attributes' => array(
      'alt' => t('Hero image'),
      'title' => t('Hero image title'),
    ),
    '#responsive_image_style_id' => 'hero',
  );