Mark applications that play audio for small panels.

Summary:
Don't limit audio playing indicator.
Display icon even if tasks are too small.
Fix for 0490f101

Test Plan:
Set panel size 20 pixels or less.
Audio indicator on a task that plays audio, should now be visible.

Reviewers: #vdg, #plasma, hein, ngraham, broulik

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

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26627