o Fix filename assembly.
  The filenames coming from the commandline were always assembled from
  the current working directory + filename given on the commandline.
  That does nto work if the filename given is an absolute path:
    openscad /home/username/foo.scad
  The used boost library does not account for that.
  Change the assembly to use qt QDir(), that does the correct thing
  platform independently.
o While working at this file: remove an unused variable and make
  an unused parameter infisible.