Don't play sound when muting
Summary: When the user presses the mute key to mute the sound they explicitely don't want sound. E.g. when they are in a public place and mute their system playing a sound is awkward
Test Plan:
On an unmuted system press mute button -> no sound played
Press mute button again -> sound is played
Reviewers: drosca, Plasma
Reviewed By: drosca
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D21045