Use pactl instead of KMix in PauseMusic Plugin

Summary: Using pulseaudio via pacmd for controlling the system volume is way easier than using KMix. Same could be done to control the system volume in the MPRIS Plugin

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: anthonyfieroni, thomasp, albertvaka, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D7922