Move axis aliases into one axis enum

[movabo]

In the future we'll need multiple constants for each axis.
We should create one enum which holds all the required constants.

[IceflowRE]

Also move BBOX_DIRECTIONS and BBOX_CORNERS_ON_FACE_INDICES from the collisions.

[IAmNotHanni]

Also move BBOX_DIRECTIONS and BBOX_CORNERS_ON_FACE_INDICES from the collisions.

I'm working on that in hanni/collision2