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.

According to https://github.com/ibm-js/sdk/blob/master/GUIDELINES.md#jshint, we allow by default override of max complexity to 15 but not more. Maybe this can me merged as is but it would be good to not start adding higher and higher complexity exceptions and so at least add a TODO comment to fix that later (and fix it now if easy).