The org.zkoss.zk.ui.util.WebAppCleanup interface
A listener could implement WebAppCleanup to cleanup a ZK application, when it is being destroyed.
When a ZK application is going to be destroyed, it invokes the
cleanup method of this interface such that developers could plug the application-specific codes to cleanup the application.
Instantiation: An independent instance of the given class is instantiated each time before the method is invoked. It means it is thread safe, and all information stored in non-static members will be lost after called.