title: RadTreeView make parent items expandable but not selectable
body: <p>My treeview hierarchy is like this,</p>
  
  <pre><code>parent 1a
      parent 2a
            child 1a
  parent 1b
       child 1b
  parent 1c
  </code></pre>
  
  <p>(In the hierarchy there are 700+ nodes. Child items gets loaded on demand to improve performance. )
  Treeview will be initially loaded with level 1 items. I want only leaf nodes (Items with no children) to be selected. (As per the example, child 1a, child 1b, parent 1c only should be selectable)
  But if an item has children, it should be expandable, but not selectable.</p>
  
  <p>Tried the "Enabled" attribute, but you can't use expand as well. Please help! </p>