Double-hull

In past the hull thickness used to be doubled by two-layer plating, but this led to the undesired construction of "mere double-hulls". Twice as heavy, but improving the maximum durability only 1.5 times compared to the single-layer plate (see http://forum.birth-of-the-empires.de/viewtopic.php?p=14532#p14532 (German)). (Source ShipDesignMenuView.cpp: Double-hull decreases Maneuverability by 1 compared to single plate. Ships with Maneuverability 0 or 9 are excluded from the scheme.)