Amend file name generation algorithm

[madig]

1. Update list of reserved file names with current Microsoft recommendations
2. Fix out-of-sync list of illegal characters
3. Point out that filenames must be valid Unicode strings (regardless of what the filesystem thinks), because they end up in typically UTF-8 encoded plist XML files.

This partly solves https://github.com/unified-font-object/ufo-spec/issues/199.

What's left:

1. Reserved filename escaping can be less aggressive that it is currently because only the part before the first period is relevant (i.e. "con" and "con.txt" but not "hello.con" and "hello.con.txt")
2. Trailing periods and spaces of the full name must be escaped for Windows filesystems.