Jakob Petsovits approved merge request !472: applets/brightness: Update applet status on dbus service changes at Plasma / PowerDevil

Watch for the DBus services register and unregister signals to updatethe service availabilty, so the UI always reflects the current status,in case something changes after initialization, or the services arestarted later than the applet

Test Plan

With the applet open, start and stop the powerdevil service vía systemctl and check how the status is updated.

powerdevil-service-update

BUG: 486067
FIXED-IN: 6.3