RadDropDownTree has a built-in property that enables easy use of checkboxes. The default value is "None" and there are three other states that can be set
to the nodes of the tree in the dropdown:
SingleCheck - Enables checking of nodes without effecting its child or parent nodes like in "CheckChildNodes" and "TriState".
CheckChildNodes - Checks all child and sub child nodes (if there are any) of a single node .
TriState - When set in the DropDownTree it allows for Nodes' CheckBoxes to have an additional,
third state called Indeterminate. A Node's CheckBox is in Indeterminate state if
it has both Checked and Unchecked CheckBoxes of child Nodes.