remove dropdown.js reliance on roles and fix keyboard navigation

[pvdlg]

Per migration documentation in 4d8d8bd:

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

In addition fix the the space key behavior on firefox for <button> elements: #21159. Thanks to @RyanThomasMusser for the analysis here. Fixes #21159.

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

With this modification the javascript now matches the markup in the documentation.
The keyboard navigation works again now in dropdowns.

Also fixes #21941 (see comment)

[pvdlg]

Update unit test with new markup.
Per comment

[jump020305]

@ALL can you help me with this one http://stackoverflow.com/questions/41645884/accessibility-for-select-fields-in-ie11

[mdo]

This PR is a little wonky right now, but any chance for a review from @Johann-S and @bardiharborow on the core changes at https://github.com/twbs/bootstrap/pull/21535/files#diff-bfe9dc603f82b0c51ba7430c1fe4c558?

[pvdlg]

I merged the last changes from v4-dev and for some reason the diff in the PR show all these changes even though it shouldn't as they are not different from the last v4-dev.
Looking at the diff between v4-dev and my dropdown-keyboard branch show the proper diff: v4-dev...vanduynslagerp:dropdown-keyboard

[Johann-S]

It's a very good work thank you @vanduynslagerp 👍

IMO we shouldn't remove dependency to role="menu" or role="listbox" for the accessibility but I'm not an expert in this subject we should call the ARIA guy @patrickhlauke 😄

[pvdlg]

@Johann-S the PR remove the dependency to role="menu" and role="listbox" in the JS selectors so it's consistent with other JS module and rely on classes for selector.
Like other components in Bootstrap, the use of role="menu", role="listbox" or aria- for accessibility is recommended in the documentation and examples.

[Johann-S]

Can you change the indent please ?

[Johann-S]

Sorry but your branch is out-of-date so can you update ?

[Johann-S]

Again but I arrived too late and your branch isn't up to date and #21802 too

[Johann-S]

Can you notice me when you update your branch ? Or can you allow me to update your branch ? Thank you

[pvdlg]

I gave you permission

[patrickhlauke]

Looks good, and does indeed fix the issue of Firefox/space. Good stuff