Separate STLs for separate parts

[arthurlutz]

When exporting the blender model to STL we get a 3 part STL where some parts are hanging in the air, it would be nice to 3 separate parts.

And by extension a part for each "extractable" part. For 3D printing we don't need (or want to avoid) printing the parts needed for injection molding manufacturing.

Will send a pull-request to show what I was able to export with blender.