applets/taskmanager: New grouped task style for themes without the normal graphic for it
This commit implements an idea in https://bugs.kde.org/show_bug.cgi?id=443235 for a different style of grouped task indicator. It is implemented to only use this new style when the active Plasma theme lacks the grouped task indicator elements in its `tasks.svgz` file. So if we want to use this in the Breeze Plasma theme, we would have to remove those graphics. BUG: 443235 FIXED-IN: 6.0