Increased drag when flying with lowered gears

Fixes #2509

As discussed in the above issue this increases atmospheric drag by 25% and disables the atmoshield if flying with lowered gears.

To avoid the self-destruct effect when lowering the wheels at high speeds in atmospheres, the wheel button is disabled (hidden) when lowering would cause a hull temperature > 0.7 (the ship explodes at 1.0).