Assemble guide at intersections

[vicdlsns]

I think it would be beneficial for those using boxes.py to be able to check where each piece goes in the final mounting, mainly in more complex objects.

For example; the labeled faces "front", "back", etc, and reference images/renders are very useful, but in my opinion some labeled lines and/or arrows drawn near the intersections and cuts, working together with the labeled faces or not, would end nearly all doubts about the mounting positions and orientations.

For example: