Image Carousel Example: Use buttons for controls and make rotation co…

…ntrol always visible (pull #1018)

Resolves issue #1007 by:
* making the stop/start rotation button always visible.
* Using button elements instead of links for the next and previous slide controls.

Also:
* Allows user to change between 2 view options: one with controls and captions overlayed on the images and one with them outside the image frames.
* Improves documentation in the accessibility features section.
* Adds regression tests.

aria-practices.html

@@ -480,11 +480,6 @@ <h4>WAI-ARIA Roles, States, and Properties</h4>
 
     <section class="widget" id="carousel">
       <h3>Carousel (Slide Show or Image Rotator)</h3>
-      <p>
-        <strong>NOTE:</strong> This is a new pattern as of the January 2019 release of WAI-ARIA Authoring Practices 1.1.
-        Please provide feedback in
-        <a href="https://github.com/w3c/aria-practices/issues/970">issue 970.</a>
-      </p>
       <p>
         A carousel presents a set of items, referred to as slides, by sequentially displaying a subset of one or more slides.
         Typically, one slide is displayed at a time, and users can activate a next or previous slide control that hides the current slide and &quot;rotates&quot; the next or previous slide into view.