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).

vehicle/DynObj.h

@@ -816,6 +816,8 @@ private:
     // zapytanie do AI, po którym segmencie skrzyżowania jechać
     int RouteWish(TTrack *tr);
     void DestinationSet(std::string to, std::string numer);
+    // blanks electronic destination sign at the end of the service; physical signs stay until changed
+    void DestinationClear();
     material_handle DestinationFind( std::string Destination );
     void OverheadTrack(float o);
     glm::dvec3 get_future_movement() const;