MK3 Y Motor holder end stop

I incorporated this idea into an overall better and more complete solution, see Taurus Y.

---

So TheZeroBeast had a brilliant idea to solve his Y axis crash recovery issue (see https://github.com/prusa3d/Original-Prusa-i3/issues/76). He submitted a pull request but no telling if/when that will make it in.

I didn't realize (should have) that the y carriage stops on the bearing on one side. There is quite a bit of range this bearing can be mounted which means the end stop varies between printers. And in the case of TheZeroBeast, it caused movement which created variation on the same printer. So his elegant answer was to extend the top of the Y motor holder to be a fixed and centered end stop that works with the Y belt block. Smart. I just added a small tab to add an optional third screw to secure the Y motor a bit better. Requires an M3x6mm (not supplied) because the motor bore is too shallow for a full 10 screw which would stop the Y carriage too early.

Hats off to TheZeroBeast. I am publishing this so folks can get his solution sooner than though GitHub.

Update 7/8: added access hole for pulley set screw

###More parts
see Taurus line of upgrades for best performance.

---

I have many parts for MK3. I like them all, but here is what I recommend to tighten things up and improve reliability and print quality. This is based on my own experience using these parts and from others using the same or similar upgrades. YMMV.
Z mounts - fix alignment, sturdier
Y endstop - fix layer shift, long term reliability
hard feet - reduce vibrations
stable pulley - stabilize belt, longevity
runner - offload frame, poor man's reverse-bowden