Redesigned Z-stage support for Vertex K8400

There are two problems with the way the original K8400 Z-stage is designed.

1. The whole Z-stage is resting on the coupler between the motor and the leadscrew. This coupler then acts as a spring and can be compressed by ~0.5mm just by pushing on the leadscrew with a finger. I see this a lot in 3D-printers and it's just bad practice. The coupler is just there for misalignment between the leadscrew and the motor axle, NOT to actually take any load in axial direction.

2. Because there are in