repeat something in a thread, but stop when the program stops

[Harald Massa]

Thank you very much, Peter and Tim!

I was really not aware of the timeout-parameter of Queue and did not 
understand demons!

> Measure it:  you won't be able to see a difference.  Modern processors
> cram on the order of 1E9 cycles into each second.  Doing a tiny amount
> of work 20 times per second is trivial.

You are surely right, Tim. Measuring did not detect the difference. I just 
see my tasktray collecting more and more icons of programs which just do a 
very tiny amount of work every half a second, and ... as them getting many, 
the computer begins to feel more sluggish.

Harald