introduce `type="list"/"filter"` for a group in samplelst term

usecases:
1. custom samplelst term built by filterui from group tab should use type=filter, and assign the filter obj to the group, instead of flat sample lst
2. samplelst term can continue to use type=list with flat sample list at rest of places e.g. scatter lasso, matrix drag select, even mds3 tk disc select