Improved Smooth X-axis for Prusa i3 with Leadscrews (adds Adjustable Backlash and Clearance)

[Smooth X-axis for Prusa i3 with Leadscrews](https://www.thingiverse.com/thing:1103976) is a really nice design. The only drawback that I have seen with this x-axis is that you cannot adjust the clearance of the bearings. After switching to drylin bearings, the clearance was too larger to make good prints. This was the motivation to create this remix.
Not really related to this design, I have also added adjustable anti backlash nuts to for the z-leadscrew. I use this design in [Retroprinter](ht