KeyNav: add focusChildren=false mode for ComboBox

refs ibm-js/deliteful#331
Ah right, I meant to change the name to `d-active-descendant`.

Given the complications you mentioned with setting up generic CSS rules, I think it makes sense for ComboBox to define the CSS rules, something like

```css
.d-combo-box-list .d-active-descendant {
   background: blue;
   color: white;
```