[AdminListBundle] Make it possible to have no item actions for some records

[jockri]

When some rows in the adminlist do not need to have a certain item action, you can just do this:

        $create_route = function ($item) {
            if (!$item->foo()) {
                return null;
            } else {
                return array(
                    'path'   => 'mybundle_admin_projects',
                    'params' => array('id' => $item->getSlug())
                );
            }
        };
        $ia = new \Kunstmaan\AdminListBundle\AdminList\ItemAction\SimpleItemAction($create_route, "eye-open", "Preview");
        $this->addItemAction($ia);