Fix secret/vehicle checks

[lahm86]

• Updated each secret location that depends on a vehicle to add the vehicle type.
• If randomizing vehicles and a secret exists in the level that has a dependency on that vehicle type, the vehicle location will not be randomized. This avoids issues with the path from the vehicle to the secret being impossible, the idea being that the secret is placed based on the vehicle's original position.
• Tidied up some of the logic to test locations, and to spawn/move skidoos.

Resolves #550.