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/Train.h

@@ -195,6 +195,9 @@ class TTrain {
     bool initialize_button(cParser &Parser, std::string const &Label, int const Cabindex);
     // helper, returns true for EMU with oerlikon brake
     bool is_eztoer() const;
+    // helper, returns true if both horn tones are operated with the same lever, without separate low and high tone buttons
+    bool has_single_horn_lever() const {
+        return ggHornButton.SubModel != nullptr && ggHornLowButton.SubModel == nullptr && ggHornHighButton.SubModel == nullptr; }
     // locates nearest vehicle belonging to the consist
 	TDynamicObject *find_nearest_consist_vehicle(bool freefly, glm::vec3 pos) const;
     // mover master controller to specified position