Gets overview form header.
array Header array definition as expected by theme_tablesort().
public function overviewFormHeader($type) {
$header = array(
'title' => array(
'data' => $this
->t('Title (in source language)'),
),
'config_id' => array(
'data' => $this
->t('Configuration ID'),
),
);
$header += $this
->getLanguageHeader();
return $header;
}