message: Change savegame save code to write a numeric ID for a structure instead of a string literal
  Closes
diff: @@ -403,7 +403,7 @@ void serializeStructure(XmlElement* _ti, Structure* structure, Tile* _t)
  _ti->attribute("forced_idle", structure->forceIdle());
  _ti->attribute("disabled_reason", static_cast<int>(structure->disabledReason()));
  _ti->attribute("idle_reason", static_cast<int>(structure->idleReason()));
  - _ti->attribute("type", structure->name());
  + _ti->attribute("type", structure->structureId());
  _ti->attribute("direction", structure->connectorDirection());
  const auto& production = structure->production();
  