Summary:
Add a slider for every sink to the MPRIS Activity. The sink can be muted by clicking on the volume icon.
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See D7992
BUG: 377319

Reviewers: #kde_connect, mtijink, albertvaka

Reviewed By: #kde_connect, albertvaka

Subscribers: albertvaka, MatMaul, kdeconnect, Murz, mtijink, #kde_connect

Tags: #kde_connect

Maniphest Tasks: T4659

Differential Revision: https://phabricator.kde.org/D7993