KeyNav: add focusChildren=false mode for ComboBox

refs ibm-js/deliteful#331
I was thinking that you would set the baseClass or class of your List widget instance to something like d-combo-box-list.   As I think you mentioned, using a selector like `.d-listPopup .d-active-descendant` could affect more nodes than we want it to.

As a side note, d-listPopup is a weird name, so I will change it to d-list-popup.