portability: make filenames unicode

portability: make filenames unicode

This fixes various problems on Windows with saving/loading files
with special characters. https://gna.org/bugs/?16109

Note that we pass unicode strings to os.* functions to get
back unicode filenames (instead of the encoding of the system).

brush names: utf8 --> unicode
group names: already unicode
layer names: utf8 --> unicode
bruspack filenames: broken --> unicode