Fix SolidMasks defined in DefCore.txt for objects with mesh graphics.

There were a few leftover checks from when SolidMasks could only be defined for definitions with bitmap graphics.
Added SolidMaskPlane property.

If an object moves and other objects are attached to its SolidMask, only objects in front of this plane are moved along with it. Defaults to Plane if zero.
Cleaned up airship: Extra graphics object and unnecessary timers removed.