[applets/taskmanager] Show PA-related features even when audio indicators are disabled

Summary:
There is a checkbox in the Task Manager settings with the text "Mark applications that
play audio". Unchecking it does much more than no longer marking applications that play
audio though; it disabled all PulseAudio-related features. This not only makes the label
inaccurate, but it also seems undesirable; a user could quite reasonably wish to disable
the audio indicators while still using PulseAudio-related functionality through the Task
Manager (e.g. muting an app from the context menu).

This patch makes the setting in question really and truly only show and hide the audio
indicator icon on the Task; other PA-related functionality is always loaded when
available.

Test Plan:
Open an app or window that can play audio and start playing audio with it
Make sure that audio indicators are visible
Right-click on the task and see that there's {nav Mute} item (provided by PA
Configure the Task Manager and disable audio indicators
See that the audio indicator icon is gone from the task
Right-click on the task and see that the {nav Mute} item is still there
Configure the Task Manager again and re-enable audio indicators
See that the audio indicator icon re-appears on the task

BUG: 418164
FIXED-IN: 5.18.3

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27684