Adds an item to the translation job.
string $plugin: The plugin name.
string $item_type: The source item type.
string $item_id: The source item id.
\Drupal\tmgmt\JobItemInterface The job item that was added to the job or FALSE if it couldn't be saved.
\Drupal\tmgmt\TMGMTException On zero item word count.
public function addItem($plugin, $item_type, $item_id);