Add a setting for train speed adaptation

Prior to this change, without realistic breaking, trains would continuously run into the train in front of them. This makes them adjust their speed based on trains in front of them

src/settings_gui.cpp

@@ -1923,6 +1923,7 @@ static SettingsContainer &GetSettingsTree()
 				physics->Add(new SettingEntry("vehicle.train_braking_model"));
 				physics->Add(new SettingEntry("vehicle.train_slope_steepness"));
 				physics->Add(new SettingEntry("vehicle.wagon_speed_limits"));
+				physics->Add(new SettingEntry("vehicle.train_speed_adaption"));
 				physics->Add(new SettingEntry("vehicle.freight_trains"));
 				physics->Add(new SettingEntry("vehicle.roadveh_acceleration_model"));
 				physics->Add(new SettingEntry("vehicle.roadveh_slope_steepness"));