Geeetech i3 Electronics Cover

I wanted to fix a few issues with the original design plus add the possibility to use a `120mm` fan.

Once I was there I also added a few more vents and an additional cable exit for the bed heater.

**WARNING**
I haven't tested the `120mm` designs yet, so feedback is welcome.

**UPDATE**
After receiving some feedback via comments here I've added two additional models for the `120mm` fan using a _classic_ air vent design: the honeycomb grid seems to increase the fan noise sensibly.


**UPDATE**
After some more feedback I've added a _revision B_ which reduces the grid fins on `120mm` format but, most importantly also adds captive nut slots so that threaded inserts are no longer required: you can now choose to use an M3 nut or use a threaded insert.

In addition, in revision B the `60mm` fan position has been moved, to reduce interference with cables running inside the box, and a `40mm` version has been added for those willing to use the stock fan.

To recap:

* classic