Preserve order of APIC frames relative to each other when saving.

[obskyr]

At the time of writing, Mutagen reorders APIC frames upon save without the user being able to influence the order, which causes issues with many players (including iTunes – which is Mutagen’s main compatibility target for MP4 files, though I’m not sure whether it is for ID3 tags) as discussed in issue #436. This implements the closest to a consensus reached in #436: APIC frames are stored last, and not reordered.

This pull request conflicts with #539, as both modify the ID3 sort_key function – so allow me to provide a merged version of that as well:

        def sort_key(items):
            i, (frame, data) = items

            if frame.FrameID == "APIC":
                # The order among APIC frames is preserved, as their order
                # can influence, say, what image is chosen as cover image
                # in many players.
                secondary_key = i
                frame_key = frame.HashKey
            elif frame.FrameID == "CHAP":
                # Chapters are ordered by their `start_time`, as this is
                # also significant in many players.
                secondary_key = frame.start_time
                frame_key = frame.FrameID
            else:
                secondary_key = len(data)
                frame_key = frame.HashKey

            return (get_prio(frame), secondary_key, frame_key)

[phw]

Thanks a lot for this. I'm very happy this will be fixed.

The code looks good and clean. I'd like to give this a short test run before merging, will do so later.