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.cpp

@@ -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/.
+*/
+
+#include "stdafx.h"
+#include "vehicle/horn.h"
+
+namespace horn {
+
+int pressed(int const Signal, int const Tone, bool const Singlelever) {
+	auto const other{Tone == low ? high : low};
+	return (Signal | Tone) & (Singlelever ? ~other : ~0);
+}
+
+int released(int const Signal, int const Tone) {
+	return Signal & ~Tone;
+}
+
+} // namespace horn