Add setting to limit length of continuous inclined roads built by towns

Default to max 4 tiles

src/settings_gui.cpp

@@ -2192,6 +2192,7 @@ static SettingsContainer &GetSettingsTree()
 				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.town_max_road_slope"));
 				towns->Add(new SettingEntry("economy.found_town"));
 				towns->Add(new SettingEntry("economy.town_cargogen_mode"));
 				towns->Add(new SettingEntry("economy.town_cargo_scale_factor"));