Cache the Instances of XamlModelProperty in a Dictionary.

This is, that we have for one Property only one Instance. With this, you can open the Flat Collection Editor, and add Columns or Rows to a Grid via the Designer, and the
List in the FlatCollectionEditor is Refreshed!

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.Designer/Project/Xaml/XamlModelPropertyCollection.cs

@@ -4,22 +4,29 @@
 using System;
 using System.Windows;
 using ICSharpCode.WpfDesign.XamlDom;
+using System.Collections;
+using System.Collections.Generic;
 
 namespace ICSharpCode.WpfDesign.Designer.Xaml
 {
 	sealed class XamlModelPropertyCollection : DesignItemPropertyCollection
 	{
 		XamlDesignItem _item;
+		Dictionary<string, XamlModelProperty> propertiesDictionary = new Dictionary<string, XamlModelProperty>();
 		
 		public XamlModelPropertyCollection(XamlDesignItem item)
 		{
 			this._item = item;
 		}
 		
-		
 		public override DesignItemProperty GetProperty(string name)
 		{
-			return new XamlModelProperty(_item, _item.XamlObject.FindOrCreateProperty(name));
+			XamlModelProperty property;
+			if (propertiesDictionary.TryGetValue(name, out property))
+				return property;
+			property = new XamlModelProperty(_item, _item.XamlObject.FindOrCreateProperty(name));
+			propertiesDictionary.Add(name, property);
+			return property;
 		}
 		
 		public override DesignItemProperty GetAttachedProperty(Type ownerType, string name)