Customizer: Improve accessibility of Add Menu Items button and its panel.

props afercia, ocean90.
fixes #32682.
Built from https://develop.svn.wordpress.org/trunk@32848


git-svn-id: http://core.svn.wordpress.org/trunk@32819 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-nav-menus.js

@@ -1800,8 +1800,10 @@
 				}
 
 				if ( ! $( 'body' ).hasClass( 'adding-menu-items' ) ) {
+					$( this ).attr( 'aria-expanded', 'true' );
 					api.Menus.availableMenuItemsPanel.open( self );
 				} else {
+					$( this ).attr( 'aria-expanded', 'false' );
 					api.Menus.availableMenuItemsPanel.close();
 					event.stopPropagation();
 				}

wp-includes/class-wp-customize-control.php

@@ -1538,7 +1538,7 @@ class WP_Customize_Nav_Menu_Control extends WP_Customize_Control {
 	 */
 	public function content_template() {
 		?>
-		<button type="button" class="button-secondary add-new-menu-item">
+		<button type="button" class="button-secondary add-new-menu-item" aria-expanded="false" aria-controls="available-menu-items">
 			<?php _e( 'Add Items' ); ?>
 		</button>
 		<button type="button" class="not-a-button reorder-toggle">

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.3-alpha-32847';
+$wp_version = '4.3-alpha-32848';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.