Make CheckComboBox example simpler

Elwig/Windows/TestWindow.xaml.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Linq;
 using System.Windows;
 using Xceed.Wpf.Toolkit.Primitives;
 
@@ -10,9 +10,7 @@ namespace Elwig.Windows {
         }
 
         private void OnItemSelectionChanged(object sender, ItemSelectionChangedEventArgs e) {
-            var selected = new List<string>();
-            foreach (var item in MyComboBox.SelectedItems) selected.Add((string)item);
-            MyText.Text = string.Join(", ", selected);
+            MyText.Text = string.Join(", ", MyComboBox.SelectedItems.Cast<string>());
         }
     }
 }