Contao Custom Operation

$GLOBALS['TL_DCA']['tl_page']['list']['operations']['clearcomments'] = array(
    'label'               => &$GLOBALS['TL_LANG']['tl_page']['clearcomments'],
    'href'                => 'act=clearcomments',
    'icon'                => 'visible.gif',
    'attributes'          => 'onclick="Backend.getScrollOffset();"',
    'button_callback'     => array('WikiContentAccess', 'showActionButton')
);