Improved Bondtech BMG + E3D V6 + BLTouch carriage for BLV MGN12

**UPDATE 2020-07-11:** I have added a new V4 part cooling duct that is more compact than the V3, and can be printed much more easily. Print in PETG or ABS at 0.2mm layer height, no support needed.
Also, I have updated the V3 carriage-side wire loom adapter to be stronger after breaking an earlier revision on my printer.

**UPDATE 2020-06-02:** For better cable management I have added new V3 versions of the wire loom adapters (WireLoomAdapterV3.stl and MGN_XMotorCoverV3.stl) and some clips. Th