NEW feature hidden extrafields

htdocs/core/actions_extrafields.inc.php

htdocs/core/class/commonobject.class.php

htdocs/core/class/extrafields.class.php

htdocs/core/tpl/admin_extrafields_add.tpl.php

htdocs/core/tpl/admin_extrafields_edit.tpl.php

htdocs/core/tpl/admin_extrafields_view.tpl.php

htdocs/core/tpl/extrafields_view.tpl.php

htdocs/install/mysql/migration/3.9.0-4.0.0.sql

htdocs/install/mysql/tables/llx_extrafields.sql

htdocs/langs/en_US/admin.lang