make construction its own system (extracted from BuildingSystem)

src/lib/sim/Simulation.h

@@ -25,6 +25,7 @@
 
 class AiSystem;
 class BuildingSystem;
+class ConstructionSystem;
 struct Command;
 class Hasher;
 class CombatSystem;
@@ -218,6 +219,7 @@ private:
     FactoryState                      m_factoryState;
     BeltSystem                        m_beltSystem;
     std::unique_ptr<BuildingSystem>   m_buildingSystem;
+    std::unique_ptr<ConstructionSystem> m_constructionSystem;
     std::unique_ptr<ShipSystem>       m_shipSystem;
     std::unique_ptr<AiSystem>         m_aiSystem;
     std::unique_ptr<MovementIntentSystem> m_movementIntentSystem;