Better extruder dotation visualizer

This is better version of extruder rotation visualizer by Mikoláš Zuza. I had trouble with CA-glued magnets not sticking to the printed part. So I remixed the model by adding sleeve for friction fit of the 8x4 magnet. Additional 8x4 magnet is then used to mount to the stepper rotor.

I also enlarged it a bit, from 40 to 45 mm diameter, because I think it looks better.