Changed '/' to os.sep to make the Bieri GUI OS independent.
This corresponds to one component of the patch_II file attached to https://gna.org/task/?6847 by
Michael Bieri (https://gna.org/users/michaelbieri). This change was discussed in the tread starting
at https://mail.gna.org/public/relax-devel/2010-01/msg00007.html.