KeyNav: add focusChildren=false mode for ComboBox

refs ibm-js/deliteful#331
Oh, and I set aria-activedescendant on the root node in anothercommit, so you should be able to make a CSS rule like

```css
d-list[aria-activedescendant] .d-active-descendant {
color: white;
background: blue;
}
```