MK3 screw-on reverse bowden

# MK3 quick screw-on reverse bowden + connectors

A compact, fully 3d-printed reverse bowden setup compatible with the MK3 or the MK3 Bondtech extruder upgrade.

I designed this after trying out PC4-M6/10 connectors, which I didn't like for the following reasons:

- Straight PC4 connectors cannot be really used without a tube inserted, since the internal blades will oppose the filament moves during retraction. These are fine if you plan to use the reverse bowden all the time, but I wanted something optional without having to swap the PTFE tube with a short section all the time.

- PC4-M6/10 connectors fitted to a screw-on design become unnecessarily tall.

- Twisting the screw with a PC4 connector while holding the tube stationary eats the PTFE tube at the blades. I had a PC4 connector fail due to this.

In this design the bowden attachment is fully optional and can be quickly screwed-on only when required. Fits regular 4mm bowden tubing. Includes a female-female bowden con