Hide redundant confirmation dialog exit button for assistive technologies.

HFJ-2004

js/h5p-confirmation-dialog.js

@@ -120,6 +120,7 @@ H5P.ConfirmationDialog = (function (EventDispatcher) {
     // Exit button
     var exitButton = document.createElement('button');
     exitButton.classList.add('h5p-confirmation-dialog-exit');
+    exitButton.setAttribute('aria-hidden', 'true');
     exitButton.title = options.cancelText;
 
     // Cancel handler