Modify front clasps to prevent loosening/prying

Modify front clasps _(which rotate to hold down the top-plate)_ to
prevent them loosening.  This is accomplished by adding a ridge that the
foot of each clasp slides under to prevent the clasp from prying upwards
due to the force from the top-plate pushing up.
Rename SketchUp files to use unix-friendly names
Combine all parts into single sketchup model and add stl/svgs