add relativepoint tostring

src/Avalonia.Visuals/RelativePoint.cs

@@ -177,5 +177,16 @@ namespace Avalonia
                     unit);
             }
         }
+
+        /// <summary>
+        /// Returns a String representing this RelativePoint instance.
+        /// </summary>
+        /// <returns>The string representation.</returns>
+        public override string ToString()
+        {
+            return _unit == RelativeUnit.Absolute ?
+                _point.ToString() :
+                 string.Format(CultureInfo.InvariantCulture, "{0}%, {1}%", _point.X * 100, _point.Y * 100);
+        }
     }
 }