Y Belt holder + tightener

This is a replacement for Y belt holder on rebel2/rebelix printers, should probably fit other similar prusa-like machines. The point of making this is to be able to tighten the belt with a screw just right so the belt doesn't get damaged further and to hopefully keep the printer from making horrible sounds during fast Y-axis movements.

Sliders are designed for GT2 belt, I might make different variants on request.