Dondolo Rework v2

Video: https://youtu.be/erD9FEEfzTU

### Problem statement ###

This is a rework of the original Dondolo v1.01b extruder design by Gianni3D. After construction and use of this extruder I was facing an issue with poor repeatability in positioning of hotends after switching. This was cased by specific design of the idler parking mechanism. Since the pendulum is pushing the parking idler away it is always under stress, and it bends under the force from the parked idler. This leads to inaccuracy in hotends’ positioning (at least in my case).

This rework brings following improvements:
* It uses more lightweight servo HS-225 BB with nylon bearings for switching of hotends.
* Both the pendulum and the idlers are now actuated by the servo horn.
* The pendulum can be freely moved by the servo horn and it is not under stress in the end positions. It holds in the end positions only by the force generated by neodymium magnets.

Also, different to the original design, this extruder uses