Use common interface for object reference in bulk actions
- use same interface in child-resources and form-usages (when bulk-actions are used)
- move "get_ids_for_bulk_actions" from page to package (to make in usable in www/admin)
- use consistently "get_ids_for_bulk_actions" (also in www/admin/*.tcl)
- provide backward compatibility (still support "name" and "-parent_id")