KeyNav: add focusChildren=false mode for ComboBox

refs ibm-js/deliteful#331
This looks as a doc mistake (inconsistency with the code), since the name of the CSS class set by the code is `d-active`. That said, I wonder if this name isn't  too generic (shouldn't be more specific). No strong opinion on that, though (and the section "CSS class naming" of https://github.com/ibm-js/sdk/blob/master/GUIDELINES.md is subject to various interpretations, similar to the event naming rules).
Now, if the name stays generic (`d-active`), while the class is actually NOT defined by delite, it would mean List (or ComboBox?) would define it. Given that the popup is (currently) NOT contained in the widget (in the DOM hierarchy sense), we'd need to define the CSS class in a manner which would only hold for popups of the ComboBox (to avoid affecting other widgets using it, if any would ever exist). Thus it would use for instance a selector based on `d-listPopup`, which is a class name dynamically built by https://github.com/ibm-js/delite/blob/master/popup.js#L262. This could work, but it means the CSS of a widget would make assumptions about the internals of delite/popup... All in one, no big deal, but I wonder if we shouldn't stick with one of these options:
1. Keep the generic name (`d-active`) but define the class in delite, or
2. Make the name more specific and let List/ComboBox define the class.
On this point, these are just brainstorming cents.