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/Trees/ContentTreeController.cs

@@ -109,7 +109,7 @@ namespace Umbraco.Web.Trees
 
         protected override MenuItemCollection PerformGetMenuForNode(string id, FormDataCollection queryStrings)
         {
-            if (id == Constants.System.Root.ToInvariantString())
+            if (id == Constants.System.RootString)
             {
                 var menu = new MenuItemCollection();