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();
 				}