Add feature: realistic train braking

Add setting to select train braking model.

src/settings_gui.cpp

@@ -1812,6 +1812,7 @@ static SettingsContainer &GetSettingsTree()
 			SettingsPage *physics = vehicles->Add(new SettingsPage(STR_CONFIG_SETTING_VEHICLES_PHYSICS));
 			{
 				physics->Add(new SettingEntry("vehicle.train_acceleration_model"));
+				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.freight_trains"));