The org.zkoss.zk.ui.util.ExecutionCleanup interface
A listener could implement ExecutionCleanup to cleanup an execution that is being destroyed.
When ZK Loader is going to destroy an execution, it invokes the
cleanup method of this interface such that developers could plug the application-specific codes to cleanup an execution.
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.