Swapped all System.Root string conversions

to use the `System.RootString` constant.

This saves on integer parsing and reduces the number of
new strings being created.

Since the `System.Root` is `-1`, it didn't need the culture invariant
conversion.

src/Umbraco.Web/Models/Trees/TreeRootNode.cs

@@ -26,7 +26,7 @@ namespace Umbraco.Web.Models.Trees
     [DataContract(Name = "node", Namespace = "")]
     public sealed class TreeRootNode : TreeNode
     {
-        private static readonly string RootId = Core.Constants.System.Root.ToString(CultureInfo.InvariantCulture);
+        private static readonly string RootId = Core.Constants.System.RootString;
         private bool _isGroup;
         private bool _isSingleNodeTree;