Mechanical Filament Runout Sensor for Prusa MK2, MK2S

New version of the filament sensor from NSGrimace using the snap-on design from CNCKitchen. As optical sensors usually have problems with translucent and transparent filaments, a mechanical microswitch is a good solution to reliably check for the filament.

I wanted the sensor to be as small as possible without using screws. I added several improvements to the design:
- The housing does not touch the standard idler
- Screwless cover and microswitch mounting


The switch is monitored by Oc