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

The dojox/form/manager/_Mixin module blindly assumed a _Widget superclass, depending 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.
Merge branch 'ticket-17173' of http://github.com/bitpshr/dojox into ticket-17173