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/horn.h

@@ -0,0 +1,24 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+// the horn controls of the cab, on the WarningSignal bits of the vehicle (1: low tone, 2: high tone)
+namespace horn {
+
+int const low{1};
+int const high{2};
+
+// the signal after the key or button of the Tone is pressed. a single lever for both tones, without
+// separate buttons for them, can't sound both tones at once
+int pressed(int Signal, int Tone, bool Singlelever);
+// the signal after the key or button of the Tone is released
+int released(int Signal, int Tone);
+
+} // namespace horn