Scale issues on exporters/addons

Rework and refine the scale on various addons and exporters. Quite often people tell me that my STLs, FBXs or STEPs are 100x, 1000x too small. Also, the Bolt addon embedded in Blender hasn't been updated since years, and creates 3 meters tall screws. For consistency, I think we should assume that 1BU is 1 Meter, and if not the scale slider on exporters will be a workaround, not the other way around