Customizer: Accessibility enhancements.

* Move tabindex/focus code into `onChangeExpanded` so that it works for cases where `expand()` is called directly.
* Use `visibility: hidden` to hide hidden elements from keyboard focus and screen readers.

props celloexpressions.
fixes #31336.
Built from https://develop.svn.wordpress.org/trunk@33069


git-svn-id: http://core.svn.wordpress.org/trunk@33040 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.3-beta1-33068';
+$wp_version = '4.3-beta1-33069';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.