Space-separated list of CSS style class(es) to be applied to the tree node labels.
MethodExpression representing an tree node toggle listener method that will be notified when the tree node
is toggled. The expression must evaluate to a public method that takes an TreeToggleEvent parameter, with a
return type of void, or to a public method that takes no arguments with a return type of void.
In the latter case, the method has no way of easily knowing where the event came from, but this can be
useful in cases where a notification is needed that "some action happened".