[Task Manager] Toggle mute when the audio indicator is clicked

Summary:
The task manager shows an indicator on tasks playing audio (when
enabled in the settings). This patch adds the ability to mute/unmute
an application playing audio by clicking on the audio indicator in
the task manager.

Test Plan:
* Check "Mark applications that play audio" in the Task Manager Settings
* Open an application that plays audio
* Click on the audio indicator and it should toggle the mute state
{F7324364}

Reviewers: #plasma, #vdg, ngraham, filipf, hein, faridb

Reviewed By: #plasma, #vdg, ngraham, hein

Subscribers: GB_2, mart, hein, filipf, mvourlakos, ngraham, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19822