System tray dock widget: Fix showing the icon for an error condition
For example, if no devices are present. The Plasma system tray cannot use a private application icon, so use the standard volume indicator with an error overlay. Painting the overlay explicltly because KStatusNotifierItem::setOverlayIconByName() does not appear to work and KIconThemes is no longer available.
Showing with 81 additions and 54 deletions