title: gong-wpf-dragdrop copy to target
body: <p>I'm using gong-wpf-dragdrop to handle drag and drop in wpf.<br/> But whenever I drag items in source listbox and drop to another target listbox, these items were deleted in source listbox.
  How to keep items in source listbox after drag and drop these to another target listbox?</p>
  
  <pre><code>&lt;ListBox x:Name="list1"
                   dd:DragDrop.IsDragSource="True"
                   dd:DragDrop.IsDropTarget="True"&gt;
              &lt;TextBlock&gt;1&lt;/TextBlock&gt;
              &lt;TextBlock&gt;2&lt;/TextBlock&gt;
          &lt;/ListBox&gt;
          &lt;ListBox x:Name="list2"
                   dd:DragDrop.IsDragSource="True"
                   dd:DragDrop.IsDropTarget="True"&gt;
              &lt;TextBlock&gt;3&lt;/TextBlock&gt;
              &lt;TextBlock&gt;4&lt;/TextBlock&gt;
          &lt;/ListBox&gt;
  </code></pre>
  
  <p>This is an example. I want to keep a copy of Item "2" in list 1 after drop it into list 2</p>