Customize: Fix ability to shift-click on placeholder/pre-saved nav menu items in preview to focus on corresponding control.

Props westonruter.
Merges [39562] to the 4.7 branch.
Fixes #39102.

Built from https://develop.svn.wordpress.org/branches/4.7@39572


git-svn-id: http://core.svn.wordpress.org/branches/4.7@39512 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/customize-preview-nav-menus.js

@@ -419,7 +419,7 @@ wp.customize.navMenusPreview = wp.customize.MenusCustomizerPreview = ( function(
 				return;
 			}
 
-			navMenuItemParts = $( this ).attr( 'class' ).match( /(?:^|\s)menu-item-(\d+)(?:\s|$)/ );
+			navMenuItemParts = $( this ).attr( 'class' ).match( /(?:^|\s)menu-item-(-?\d+)(?:\s|$)/ );
 			if ( navMenuItemParts ) {
 				e.preventDefault();
 				e.stopPropagation(); // Make sure a sub-nav menu item will get focused instead of parent items.