ec signal lamp: allow turning off the signal lamp

fix calculation of average power on the signal lamp

should fix
#46
and
https://bugs.kde.org/show_bug.cgi?id=455968

src/electronics/components/ecsignallamp.cpp

@@ -60,7 +60,7 @@ void ECSignalLamp::stepNonLogic()
     if (advanceSinceUpdate == 0) {
         advanceSinceUpdate = 1; // do not try to divide by 0
     }
-    avgPower = fabs(avgPower * advanceSinceUpdate + (voltage * voltage / RESISTANCE)) / advanceSinceUpdate;
+    avgPower = fabs(avgPower * 0.99 + (voltage * voltage / RESISTANCE) * 0.01);
     ++advanceSinceUpdate;
 }