Avoid garbling the sound if the volume slider is moved fast

Summary:
The audio should not be garbled if you move fast the audio slider  (it distorts because it stops and plays the feedback sound every few milliseconds)

An alternative approach is to `return`; if the `ca_context_playing` says that the sound is still playing, but that gives less "resolution"

BUG: 409525
BUG: 410716
FIXED-IN: 5.17.0

Reviewers: #plasma, drosca, davidedmundson, ngraham

Reviewed By: #plasma, davidedmundson, ngraham

Subscribers: anthonyfieroni, filipf, ngraham, davidedmundson, plasma-devel, kezik

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22316