Getting podkit to work on an iPod nano 7th generation

[Pred8or]

Hi! I have an iPod nano 7th generation (firmware 1.0.4) and wanted podkit to sync it. Out of the box podkit only reads it. It turned out the nano 6G/7G need three things before the firmware accepts a library written by libgpod: the HashAB signature, the hidden "category" playlists plus iTunes' SQLite schema level, and keeping the play data the device records itself.

I got this working and tested it on my nano 7G: adding and removing tracks, playlists, ratings, play counts kept across syncs, videos, and composer/sort fields. I split it into eight small stacked PRs so each part can be reviewed on its own:

1. three small macOS fixes (independent of the rest)
2. the HashAB signer (vendored from the Unlicense project dstaley/hashab), devices stay read-only
3. category playlists and schema level
4. keeping the device's play data, and enabling sync for the nano 7G
5. a nano 7G video profile and two video fixes
6. --custom-bitrate and MP3 ABR
7. composer, grouping, totals and sort fields to the iPod
8. the same fields in file tags and from OpenSubsonic

Happy to reshape, squash or drop parts if something doesn't fit podkit's direction. The nano 6G is only covered by synthetic tests, so it stays read-only.

PRs: #76 · #77 · #78 · #79 · #80 · #81 · #82 · #83