Anet A8-B v1: X-Belt Tensioner

I did not get the original by FredGenius to print properly on my A8, so I drew a version with looser tolerances.

I changed deliberately:
- Tooth are much looser in the belt gripper (I used washers to fixate the belt after insertion)
- Dove tail with wider tolerance and 55° angle instead of 45°
- 3mm tightener screw instead of 4mm (just because I had those available)
- tightener screw closer to the belt to reduce skew forces

(all other changes just happened.)

You'll need:
**3mm x 30