Fix adding two times the same string in the Designer:

This does not work before:
                            var addItem = page.designSurface.DesignContext.Services.Component.RegisterComponentForDesigner(se.Value);

                            addItem.Key = se.Key;
                            d.Properties["Hardware"].CollectionElements.Add(addItem);

maybe we also need to change for Value Types!

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

@@ -89,7 +89,8 @@ namespace ICSharpCode.WpfDesign.Designer.Xaml
 			}
 			
 			XamlDesignItem item = new XamlDesignItem(_context.Document.CreateObject(component), _context);
-			_sites.Add(component, item);
+			if (!(component is string))
+				_sites.Add(component, item);
 			if (ComponentRegistered != null) {
 				ComponentRegistered(this, new DesignItemEventArgs(item));
 			}