Fixed MaxListenersExceededWarning when creating many progress bars.

[TychoTheTaco]

When you create many progress bars (>10), eventually you will get MaxListenersExceededWarning since SIGINT and SIGTERM listeners keep getting added in the constructor but are never removed. I fixed it by adding them on start()/create() and removing them on stop()

[AndiDittrich]

Hi @TychoTheTaco

thanks for your contribution. Indeed that' a possible trap within the current implementation which was not tested - sry.
You solution may cause other issues by adding multiple handlers to the event stack, therefore it requires some modification before merging

[TychoTheTaco]

I've added a check to make sure the callback only gets added if the bar is starting for the first time.

[AndiDittrich]

i've applied a slightly modified version of your patch