RailcoreII build chamber cable management

This provides a cover over the wiring in the build chamber of the RailcoreII printer, as well as an updated cable clip based on [this clip](https://www.thingiverse.com/thing:2812010). This keeps the cables nicely constrained and tidied up inside the build chamber.

The cable clip is a little beefier and matched to the housing's opening, and also provides a hole for the bowden tube.

FAQ: what's the small cable sleeve coming out toward the front of the printer? I have my X-homing switch in