Allow to set pane attributes from within the content type

In #2169571: Impossible to set attributes on Panels panes the standard $attributes were added to panel-pane.tpl.php. However currently it is only possible to change the attributes on a pane using template_preprocess_panels_pane(). It would be very helpful to also allow to set this from within a content type plugin itself. We can already set the CSS ID and classes alongside a host of other things, but not the attributes.

An example:

function my_content_type_render($subtype, $conf, $args, $context) {
  $content = new stdClass();
  $content->title = t('My title');
  $content->content = '<p>My content.</p>';

  // We can already set the CSS class and ID properties.
  $content->css_class = 'my-class';
  $content->css_id = 'my-id';

  // Let's also allow to set other attributes!
  $content->attributes = array(
    'data-my-data' => 9000,
    'draggable' => 'true',
    'tabindex' => 20,
  );

  return $content;