Sculptr Top Plate Remix w/ less screw holes

This is a reimplementation of [R0b1n's Sculptr Top Plate](https://www.thingiverse.com/thing:1703152) in OpenSCAD. The original STL had some mesh errors and was oriented the wrong way, it was easier to do this than to fix the mesh.

Two STLs are provided. One removes the center hole for the spool holder but leaves the holes to mount R0b1n's Bowden extruder, and one removes both the center hole and the extruder mounts, leaving a flat top panel.

I also changed the screw holes so they fit M4 pa