title: AdvTree select all nodes programmatically
body: <p>I am trying to select all nodes on Ctrl + A on an AdvTree.
  it is set to allow multiselect.</p>
  
  <p>I have the event PreviewKeyDown to check ctrl, and A are pressed.</p>
  
  <p>I tried this:</p>
  
  <pre><code>For Each nd As Node In tvComputers.Nodes
      nd.SetSelectedCell(nd.Cells(0), Nothing)
  Next
  </code></pre>
  
  <p>but its only selecting the last item in the tree, it doesnt seem to add to the selected list</p>