Locale-independent support of original Japanese filenames

[nmlgc]

As of P0217, these still require external locale emulation to work.

• [x] Loading filenames with Japanese characters independent of locale
• [ ] Bonus feature: Checking for mojibake filenames and automatically fixing them
• [ ] Bonus feature: Checking for both mojibake and Japanese filenames, showing the difference between both files, and asking the user to pick one of them

[nmlgc]

Requested by Arandui as a secondary priority for T0212, with both bonus features if funding is sufficient.