An object passed to UITreeNode (a dynamically expandable
tree node) must extend the UITreeNode class and override these methods:
isLeaf: true if it has no children;
loadChildren: inserts children into tree;
toString: for rendering itself
Creates a tree node for a plan by calling the UITreeNode constructor.
Parameters: uiPlugin - this user interface plug in Parameters: planName - name of plan to display Parameters: clusterId - cluster from which to obtain plan exception: UINoPlanException - thrown when the plan does not exist