Allow controlling vactube object rotation

[vrad-exe]

Vactube objects should have a keyvalue allowing you to configure how the object rotates while in the tube, with the following options:

• Random (current behavior)
• Face direction of motion (perhaps allow setting the "forward" direction somehow?)
• Fixed orientation (object keeps its angles set in Hammer)

This would be useful to allow making things like P1 style vactubes or bomb tubes, which typically don't have objects rotating. I'm not sure what the "face motion direction" mode would actually be used for, but it seems good to have.

If objects bouncing off the sides of the tube is ever implemented, that should probably also be selected through this keyvalue.

[TeamSpen210]

I really need to redo the rotation anyway.