The graphics.fetch_icon() function can now return either the absolute or relative path to the icon.

This is a partial solution for bug #21042 (https://gna.org/bugs/?21042).
Fix for bug #21042 (https://gna.org/bugs/?21042).

The docs/latex/fetch_docstrings.py now asks the graphics.fetch_icon() function for the relative path
to the icon rather than the absolute path.
The fetch_docstrings.py script now asks for the Unix '/' separator through graphics.fetch_icon().

This is a final fix for bug 21042 (https://gna.org/bugs/?21042).

The graphics.fetch_icon() function now accepts the 'sep' argument.  This defaults to os.sep.  But 
the docs/latex/fetch_docstrings.py script uses the Unix '/' separator to obtain a LaTeX correct path
on MS Windows.