Also, include visible <label> as a suggestion in the accessibility
callout.

as role="menu" is a very specific (and strict) ARIA pattern for
desktop-like application menus, and our dropdowns are often used
as pure navigation dropdowns, this change abandons ARIA menus for
a more open-ended and light-weight approach
(see http://heydonworks.com/practical_aria_examples/#submenus and
http://www.w3.org/WAI/tutorials/menus/flyout/#improve-screen-reader-support-using-wai-aria)
note that in dropdown.js, switched to now target ``.dropdown-menu``
instead of ``role["menu"]`` - this also prevents bootstrap scripts
from "bleeding" into non-bootstrap components on the same page.
also removed the ``role=["listbox"]`` part, which appears to be
vestigial/unused (only place in bootstrap that uses that
role are carousels, and their key handling is done separately)

http://www.w3.org/TR/wai-aria/roles#separator

unless there's a reason why ``tabindex="-1"`` was dropped in there...

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)

Refs #11410

[skip sauce]

Closes #14951 by merging it.

Refs #14362

[skip sauce]