Replace URL special chars when folder sorting

When sorting folders into order, special chars such as $, -, _ etc were
causing sorting issues while other chars such as _ worked OK. This is
all due to the char's ref in relation to a-z, 0-9 and A-Z chars in the
char table. By replacing these URL special chars (defined in RFC 1738
specification) with a \ (the only char not allowed in a file/folder
name), we are levelling these chars out, so we can safely focus on
alphanum sorting only.