KeyNav: add focusChildren=false mode for ComboBox

refs ibm-js/deliteful#331
Okay, let's go with `d-active-descendant`. So this would hold (potentially) for any widget inheriting from KeyNav. 
But i don't think I can use a `.d-combo-box-list .d-active-descendant` selector, since the list is (currently) NOT a descendant of the combo. Or do I miss something? Unless you plan to change HasDropDown to make the popup a child of the host widget, I guess I have to use `.d-listPopup .d-active-descendant` (with the little drawback I mentioned in my previous comment).