fix: single horn lever tones, skid of a stopped vehicle, destination signs after the timetable

- a cab with only the combined horn lever (horn_bt:) and no separate tone buttons could sound the low
  and high tones at once, which the lever can't do; the tones are now exclusive there, and releasing
  the other tone's key keeps the lever in place (eu07/maszyna#392),
- a skid ends when the wheel speed crosses the vehicle speed, which never happens with wheels locked
  by the brakes once the vehicle has stopped; it stayed flagged as slipping, its braking force capped by
  the sliding adhesion, so a few such wagons could push a braked consist around (eu07/maszyna#479).
  Like the Vel > 0.1 trap of the skid start, a (nearly) stopped vehicle with wheels turning at its
  speed is no longer slipping,
- lit electronic destination signs kept showing the relation after the timetable had ended, until the
  battery was switched off (eu07/maszyna#446); they are blanked when the timetable runs out, physical
  signs stay up until changed.

What the horn keys do to WarningSignal moves out of the TTrain command handlers into vehicle/horn, which
the unit tests call; the handlers keep the gauges. Tests: tests/unit/test_horn.cpp, both kinds of cabs,
and tests/unit/test_wheelslip.cpp, a wagon which stopped in a skid on slippery rail holds on a slope
like one which stopped on dry rail (without the fix it runs away, ~70 km/h after 30 s).

McZapkie/Mover.cpp

@@ -5032,6 +5032,14 @@ void TMoverParameters::ComputeTotalForce(double dt)
 	{
 		SlippingWheels = true;
 	}
+	if (true == SlippingWheels && Vel < 0.1 // the same crude trap: (nearly) stopped vehicle...
+	    && std::abs(nrot * M_PI * WheelDiameter - V) < 0.1 / 3.6) // ...with wheels turning at its speed, i.e. locked wheels at the end of a skid
+	{
+		// locked wheels never pass the rotation speed test below once the vehicle stops, and a skidding vehicle
+		// keeps its braking force capped by the sliding adhesion, so the rest of the consist could push it around
+		SlippingWheels = false;
+		nrot = V / M_PI / WheelDiameter;
+	}
 	double temp_nrot = nrot;
 	if (true == SlippingWheels)
 	{