Refactored KernelRequestListener in AbstractKernelRequestListener, ExecuteRequestListener, InspectRequestListener and CompleteRequestListener.
AbstractKernelRequestListener defines event handler methods that receive status, stdio and display_data messages, while the others define event handler methods that receive events related to the corresponding requests.
ExecuteRequestListener now defines an on_execute_finished() method that is invoked when all events relating the execute request have been received.