Add systemvolume plugin

Summary: This plugin allows controlling the system value from a remote device.

Test Plan: Apply Android patch, open up MPRIS Activity and play with the slider

Reviewers: KDE Connect, mtijink, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: kdeconnect, apol, zhigalin, albertvaka, davidedmundson, mtijink, KDE Connect

Tags: KDE Connect

Maniphest Tasks: T4659

Differential Revision: https://phabricator.kde.org/D7992