There are included front and back slope portions attachable to and detachable from storage positions and use positions, front and back holding portions capable of engaging with engagement portions of the front and back slope portions as attached to the storage positions and holding the front and back slope portions at the storage positions, and a disengaging operation tool capable of being manually operated to a disengaging position at which the front holding portion and the back holding portion are releasable from the engagement portions of the front and back slope portions and at which the slope portions are detachable from the storage positions.