Customizer: Remove "Add a Widget" button from tab order when reordering widgets.

fixes #28890.
Built from https://develop.svn.wordpress.org/trunk@29243


git-svn-id: http://core.svn.wordpress.org/trunk@29027 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-widgets.js

@@ -1588,6 +1588,9 @@
 				} );
 
 				this.$sectionContent.find( '.first-widget .move-widget' ).focus();
+				this.$sectionContent.find( '.add-new-widget' ).prop( 'tabIndex', -1 );
+			} else {
+				this.$sectionContent.find( '.add-new-widget' ).prop( 'tabIndex', 0 );
 			}
 		},