Cleanup when node is closed

When the blockly node is being closed, it might be necessary e.g. to cleanup some stuff. The node will e.g. be closed when the flow is being deployed.

The following animation shows how to log a warning when the node is being closed:

Remark: in this animation you will see that nothing will be logged at the first deploy. Reason is that our code is not active yet at the first deploy. As soon as an input message arrives, our code will register the 'on-close' handler. From then on, the warning will be logged at every deploy.