Make sure all the workers are properly terminated when closing window

[EtienneLem]

I’m not exactly sure that when a worker job is ongoing and you close the window that the worker is actually terminated. From what I could see (hear, actually (MacBook fans)) you may have to quit the browser. It may be a browser thing, but I’d prefer to be bulletproof.

Storing the working workers into an array and terminating them on window.onunload could be a good idea, just to be safe.