Make KCM look more like applet

Summary: I was looking through changes in plasma-pa and noticed, that the KCM looks very different in contrast to the applet.

Test Plan:
Checked for mute button function on all occasions (sinks, sources, applications playing, applications recording). Checked if volume slider works as expected.

{F7446909}

Reviewers: #plasma, #vdg, ngraham, GB_2

Reviewed By: #vdg, ngraham, GB_2

Subscribers: drosca, ngraham, davidedmundson, GB_2, #plasma, plasma-devel, #vdg

Tags: #plasma, #vdg

Differential Revision: https://phabricator.kde.org/D24161