ProcessList: add support for cgroups

Summary: Signed-off-by: Topi Miettinen <toiwoton@gmail.com>

Reviewers: #plasma, hein, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20786