MMU2 Frame mount with cable groove

Added a groove between the MMU2 frame mount and the frame, to allow passing through of cables. Useful for setups that have the following LED setup installed:

https://www.printables.com/model/1812-prusa-mk2mk25mk3-led-frame-holder-mmu2-compatible

Replaces the following part from Prusa's original schematic: 

• mmu2-frame-holder.stl