Add brightness control using ddcutil lib

Summary:
Setting up [[ http://www.ddcutil.com/ | ddcutil  ]] for non-root CLI use is required first.
Please refer to [[ https://github.com/d-vogel/QMLddcutil/blob/master/README.md | https://github.com/d-vogel/QMLddcutil/blob/master/README.md]] section.

Limitations:
Not tested for multiple DDC capable monitors setup, should only change brightness of first monitor anyway.

Reviewers: broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: strobach, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5381