The overall data structure representing a tree control
that can be rendered by the TreeControlTag custom tag.
Each node of the tree is represented by an instance of
TreeControlNode.
JSP custom tag that renders a tree control represented by the
TreeControl and TreeControlNode classes.
This tag has the following user-settable attributes:
action - Hyperlink to which expand/contract actions
should be sent, with a string "${node} marking where
the node name of the affected node should be included.
images - Name of the directory containing the images
for our icons, relative to the page including this tag.