allows to exclicitly free resources and break cyclic references.
Description
An instance of this interface is used by the TreeControl to
retrieve the hierarchical outline data that is displayed in the actual control.
If you implement your own XTreeDataModel you need to notify registered
XTreeDataModelListener if your model changes after the control is created.
If this is not done correctly the TreeControl will not update the data properly.
If you do not need your own model implementation, you can also use the MutableTreeDataModel.