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

Well, no, I wasn't thinking that, because I felt this would be too intrusive. The combo uses the list, including a use-case when the List is created directly by the user. Until now I didn't thought there are strong reasons for the combo to modify the baseClass of the List instance. But if you really think I should go this way...