Remixed E3D Chimera mount Prusa i3

First of all, thanks to @Domonidod1d for uploading the step-file for his design.

I remixed his design to suit my needs:
* X-carriage holes are for M4 bolts.
* Inductive sensor hole is made for sensors with M12 thread.
* Inductive sensor bar is lowered so there is less obstruction for Chimera cooling fan.
* added mounting holes on the back for older (and clone) versions of the Chimera that don't have the mounting holes on the top.
* Increased the holes for the PTFE tube/tube coupler