Dropdown: remove dependency to role="menu", role="listbox" a and li elements => fix keyboard navigation

Created by: pvdlg

Per migration documentation in https://github.com/twbs/bootstrap/commit/4d8d8bdab4b0e2931fcf92ce30d772b05a989434:

• Rebuilt dropdown styles and markup to provide easy, built-in support for <a> and <button> based dropdown items.
• Dropdown items now require .dropdown-item.

Per https://github.com/twbs/bootstrap/issues/19034#issuecomment-179749958 role="menu" should have been dropped.

This PR:

• Remove the dependency to role="menu" or role="listbox" and rely on the class dropdown-menu instead
• Remove the dependency to the a element for the dropdown-item and rely on the class dropdown-item instead
• Remove the necessity to wrap dropdown-item in li elements

All this problems affected only the keyboard navigation in the dropdown. With this modification the javascript now matches the markup in the documentation. The keyboard navigation works again now in dropdowns.

docs/migration.md

@@ -219,6 +219,7 @@ The following variables have been removed in v4.0.0. Use the `media-breakpoint-u
 
 The responsive utility classes have also been overhauled.
 
+- The `.hidden` and `.show` classes have been removed because they conflicted with jQuery's `$(...).hide()` and `$(...).show()` methods. Instead, try toggling the `[hidden]` attribute, use inline styles like `style="display: none;"` and `style="display: block;"`, or toggle the `.invisible` class.
 - The old classes (`.hidden-xs` `.hidden-sm` `.hidden-md` `.hidden-lg` `.visible-xs-block` `.visible-xs-inline` `.visible-xs-inline-block` `.visible-sm-block` `.visible-sm-inline` `.visible-sm-inline-block` `.visible-md-block` `.visible-md-inline` `.visible-md-inline-block` `.visible-lg-block` `.visible-lg-inline` `.visible-lg-inline-block`) are gone.
 - They have been replaced by `.hidden-xs-up` `.hidden-xs-down` `.hidden-sm-up` `.hidden-sm-down` `.hidden-md-up` `.hidden-md-down` `.hidden-lg-up` `.hidden-lg-down`.
 - The `.hidden-*-up` classes hide the element when the viewport is at the given breakpoint or larger (e.g. `.hidden-md-up` hides an element on medium, large, and extra-large devices).
@@ -231,7 +232,6 @@ Note that the changes to the grid breakpoints in v4 means that you'll need to go
 ## Misc notes to prioritize
 
 - Removed the `min--moz-device-pixel-ratio` typo hack for retina media queries
-- Dropped `.hidden` and `.show` because they conflict with jQuery's `$(...).hide()` and `$(...).show()` methods.
 - Change buttons' `[disabled]` to `:disabled` as IE9+ supports `:disabled`. However `fieldset[disabled]` is still necessary because [native disabled fieldsets are still buggy in IE11](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/fieldset#Browser_compatibility).
 
 TODO: audit list of stuff in v3 that was marked as deprecated