Added "renice" dialog to set application/processes priority & scheduler

Matthieu Carteronrequested to merge
rubisetcie/plasma-systemmonitor:renice into master

Attempt to implement this feature that was on KSysGuard, and never added to System Monitor before now.

Related thread on KDEDiscuss: https://discuss.kde.org/t/finishing-plasma-6-system-monitors-priority-setter/38690/3

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