Add filename unicode normalization layer

[overheadhunter]

When sharing files between different operating systems, umlauts (as well as several other characters) might be displayed incorrectly on OS X.

This is due to different unicode normalization forms:

• Finder encodes ü as ̈ (U+0308) + u
• Most other systems simply use ü (U+00FC)

Thus an additional layer is needed, which recodes filenames according to system-specific normalization forms.