Fix dojox/form/manager/_Mixin to use getChildren if getDescendants doesn't exist, fixes #17173

`dojox/form/manager/_Mixin` blindly assumed a _Widget superclass and depended on `widget.getDescendants`. This has been fixed to use `widget.getChildren` if `widget.getDescendants` doesn't exsit, thus assuming a _WidgetBase superclass at the very least. 

As an added bonus of this fix, `dojox/form/Manager` was able to ditch the _Widget dep and use _WidgetBase instead.

cc @dylans from an earlier discussion on this one