Shutdown Steam gracefully instead of forcing/killing the process

[KKire]

Steam has a way to gracefully shutdown by using the -shutdown command parameter. That should be used instead of killing the process when "Stop Steam before import" option is selected.
More info at Valve Developer Community

I think the change needs to be made in the ensure_steam_stopped() function

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Also maybe add option to start Steam silently via -silent command parameter but that can be a separate feature request.