Dropdown aria roles updated to allow listbox

Created by: plumlee

Definition of listbox role from http://www.w3.org/TR/wai-aria/roles#listbox

A widget that allows the user to select one or more items from a list of choices.

Dropdown, when used as a replacement for a native select, fits this description. The developer can choose either the menu role or the listbox role, depending on how their site uses the component.