The source project of this merge request has been removed.
If "Show tooltips" option is checked, when hovering over some programs (like Audacious), plasmashell will freeze for several seconds and then show the thumbnail image, because the thumbnail is read very slowly (located in a network disk mounted by rclone or similar programs).
Make albumArtBackground also loaded asynchronously to fix the bug #439512 (https://bugs.kde.org/show_bug.cgi?id=439512).