[WPF] Fixed "drag image must be set before starting drag operation" e…

…xception in "TreeViews" sample

TestApps/Samples/Samples/TreeViews.cs

@@ -25,6 +25,7 @@
 // THE SOFTWARE.
 using System;
 using Xwt;
+using Xwt.Drawing;
 
 namespace Samples
 {
@@ -147,6 +148,8 @@ public TreeViews ()
 			view.DragStarted += delegate(object sender, DragStartedEventArgs e) {
 				var val = store.GetNavigatorAt (view.SelectedRow).GetValue (text);
 				e.DragOperation.Data.AddValue (val);
+				var img = Image.FromResource(GetType(), "class.png");
+				e.DragOperation.SetDragImage(img, (int)img.Size.Width, (int)img.Size.Height);
 				e.DragOperation.Finished += delegate(object s, DragFinishedEventArgs args) {
 					Console.WriteLine ("D:" + args.DeleteSource);
 				};