A trivial extension to TreeWillExpandListener, to allow listeners to be
notified if another TreeWillExpandListener vetos a pending expansion.
If a TreeExpansionListener added to an instance of TreePathSupport implements
this interface, it will be notified by the TreePathSupport if some other
listener vetos expanding a node.
This interface is primarily used to avoid memory leaks if a TreeWillExpandListener
constructs some data structure (like a TableModelEvent that is a translation
of a TreeExpansionEvent) for use when the expansion actually occurs, to notify
it that the pending TableModelEvent will never be fired. It is not of much
interest to the rest of the world.