New Z axis with Pen Auto Leveling

Completely new Z axis to the drawbot (https://www.thingiverse.com/thing:3096135): solid, precise and the servo movement forces the base to up & down (no longer need the gravity action).

New "mini PAL" (Pen Auto Leveling) using a soft spring (DIA 16,5mm) to let the pen always leveled with the paper.

Will need:
- 2x screw M3x15 or M3x20 (to fix the top)
- 1x screw M3x10 or M3x15 or even M3x20 (to fix the pen)
- 3x nut M3