Add stubs for functions to perform Booleans, and get rid of mesh
stuff, including the kd tree accelerated snap to vertex (which
should not be required if the shell triangulation performs as it
should).
Also check that a sketch is not self-intersecting before extruding
it or whatever. This is dead slow, needs n*log(n) implementation.
[git-p4: depot-paths = "//depot/solvespace/": change = 1902]
