AD5M Shelf revision

A remix of the shelf designed by JakeP3d, who doesn't seem to have a profile here but needs to be recognized. There is a 2 magnet version with or without tabs on the left side. The 3 magnet version also has two files on with tabs on the left side and one without. The 3 magnet version without tabs on the left, I moved the nub on the left over slightly for a better fit.