Add an option to configure terminating processes lingering period

Looks like the XFCE Task Manager shows (in red) terminating/dead processes for too much time (over six seconds as far as I understand).

I have a job (which spawns three processes) which runs each 2 seconds and in the XFCE Task Manager, I see four (!) copies of it, three dead (red) and one active (green and then black). That means I constantly "enjoy" 9 rows of dead processes I couldn't care less about.

This is very annoying and may lead the user to believe something is malfunctioning.

changed title from Add an option to configure dead processes lingering period to Add an option to configure terminating processes lingering period

changed the description

added 2. Feature label