Listbox role support and other enhancements for deliteful/list/List

This updates the List widget with the following capabilities:

* Support for WIA-ARIA role listbox (fix https://github.com/ibm-js/deliteful/issues/56 and https://github.com/ibm-js/deliteful/issues/73)
* Support for new selectionMode "radio"
* No more default checkmarks on selected items (accessibility issues with VoiceOver when using checkmarks), addition of samples that demonstrates how to use your own (https://github.com/ibm-js/deliteful/issues/49 can be closed)

According to my accessibility tests, https://github.com/ibm-js/deliteful/issues/46 could also be closed.

I just added a comment to your original PR.  AFAICT there is no ARIA property caled `aria-selected`.