applets/showdesktop: Give it an active indicator line
requested to merge ngraham/plasma-desktop:give-show-desktop-applet-an-active-indicator-line into master
This way it shows when its effect is active, just like the Minimize All applet does.
Not sure why it doesn't touch the edges of the panel though. Any ideas @niccolove or @teams/vdg folks?
BUG: 447998 FIXED-IN: 5.24
