Add setting for if/when towns can build tunnels

Default to allowed

src/settings_gui.cpp

@@ -2191,6 +2191,7 @@ static SettingsContainer &GetSettingsTree()
 				towns->Add(new SettingEntry("economy.town_zone_calc_mode"));
 				towns->Add(new SettingEntry("economy.allow_town_roads"));
 				towns->Add(new SettingEntry("economy.allow_town_level_crossings"));
+				towns->Add(new SettingEntry("economy.town_build_tunnels"));
 				towns->Add(new SettingEntry("economy.found_town"));
 				towns->Add(new SettingEntry("economy.town_cargogen_mode"));
 				towns->Add(new SettingEntry("economy.town_cargo_scale_factor"));