Add hash function to Gradient.

Dashboard.Common/Gradient.cs

@@ -155,5 +155,14 @@ namespace Dashboard
         {
             _stops.RemoveAt(index);
         }
+
+        public override int GetHashCode()
+        {
+            HashCode code = new HashCode();
+            code.Add(Count);
+            foreach (GradientStop item in this)
+                code.Add(item.GetHashCode());
+            return code.ToHashCode();
+        }
     }
 }