normalize unicode notebook filenames

used in comparison check for notebook name change.

Unless the filenames are normalized, unchanged names may result in false positives for a name change (e.g. OS X uses NFD on the filesystem, so u'\xfc' roundtripped to the filesystem will be u'u\u0308'), which can result in the first save of a notebook after open performing the following actions:

1. save the recently opened notebook
2. `old_name != new_name`, so name change detected
3. delete old_name (which is actually new_name), which ultimately deletes the just-saved notebook

In master, this has a symptom of the first checkpoint failing because the first save actually deleted the file, and you can't checkpoint a notebook that doesn't exist.

closes #3360