Added Thickness.ToString.

Perspex.Diagnostics/ViewModels/PropertyDetails.cs

@@ -7,9 +7,6 @@
 namespace Perspex.Diagnostics.ViewModels
 {
     using System;
-    using System.Collections.Generic;
-    using System.Linq;
-    using Perspex.Controls;
     using ReactiveUI;
 
     internal class PropertyDetails : ReactiveObject

Perspex.SceneGraph/Thickness.cs

@@ -178,5 +178,10 @@ namespace Perspex
                 return hash;
             }
         }
+
+        public override string ToString()
+        {
+            return string.Format("{0},{1},{2},{3}", this.left, this.top, this.right, this.bottom);
+        }
     }
 }