Close to its front inner corner, each side plate is provided with an extension extending transversely inwards under the central plate and carrying a pivot member adapted to co-operate with a coaxial complementary pivot member fixed under the central plate, and in that each side plate is mounted to pivot about an axis that is common to the two pivot members, which axis is substantially perpendicular to the top surface of the central plate and arranged such that, in the pivoted position, each side plate is stowed substantially under the central plate. The system further advantageously comprising a device for causing each side plate to pivot relative to the central plate while the central plate is pivoting.