Use unique html-ids with drupal_html_id()

We should use drupal_html_id() to create html-ids, otherwise there are possible conflicts, as html-ids should be unique.

File: media_gallery.module
Function: media_gallery_preprocess_field
Line: ~ 1324

    foreach ($variables['items'] as $delta => $item) {
      $variables['item_attributes_array'][$delta] = array('id' => 'media-gallery-media-' . $delta);

    }

Related issue: #1327762: Media Gallery W3C Validation fail
Maybe we need also to modify media_gallery_ajax_sort() to remove possible suffix values, generated by html_drupal_id().