2D Plotter Adapter (multicolor!)

Multicolors 2D Plotter

In action: https://youtu.be/gprN2s9aOII (drawing this item https://www.thingiverse.com/thing:3173808 on a paper)

Features:
* Easy to calibrate pen alignment and depth.
* Multicolors.
* Uses magnets (8x2mm) instead of an elastic to push the pen onto the bed.
* Uses M4 screws and hexagon nuts.
* Should be relatively easy to use the same 'magnetized pen carriage' design for other printers (see 2D_Plotter_-_Anchor_Template.step file that could be included in other design).

I included profiles for "Slic3r Prusa Edition" (that can be imported with "FileLoad Config Bundle"):
* A new printer: "2D Plotter with Pens" that support multiple extruders. If needed, the number of colors can be increased by changing the value of "GeneralCapabilitiesExtruders". Object needs to be 0.10mm tall (i.e. Max Print Height = 0.11mm).
* 2 new "print settings": "0.2mm Pen Tip" and "1.0mm Pen Tip". It's usually better to use 0.2mm.
* 2 new "filaments": "Pen" and "Pen with