Special characters are replaced with underscore when copying to FAT/NTFS

Steps to reproduce:

• Create a file named with accents or special characters, e.g. demonstração.txt
• Copy it to a FAT or NTFS partition (mounted via fstab here)
• Copy is created as demonstra____o.txt

This regression was introduced by 9f7aca2e. The problem is iterating over each character of a multibyte string (UTF-8) as if it was ascii.

My suggestion is to either you skip this treatment when g_str_is_ascii returns FALSE (this could render the fix useless) or use g_utf8_* and g_unichar_* functions.