OpenSCAD: Reintroduce LED holes

[opatut]

There used to be a version where one could see the LEDs of the charger. We should add an option to generate these holes into the main case. I would still turn this option off by default, as the holes require extra steps during assembly.

The holes could have just one layer of filament across them to make the LEDs shine through, or be completely open (compare #9). This could be another parameter.