Add support for Audiobooks (.m4b) tags to EasyMP4

[Emasoft]

Please add support for Audiobooks files (.m4b).
Those are the metadata, and currently many are not accessible via easy mutagen:

Audiobooks Metadata

Name	Tag Name	FFmetadata	MP4 atom	Mp3tag	ID3v2
Book title	Title	title	©nam	TITLE	TIT2
Book title (multi-file)	Album	album	©alb	ALBUM	TALB
Chapter title (multi-file)	Title	title	©nam	TITLE	TIT2
Book sort-title	Sort-title	sort_name (title-sort)	sonm	TITLESORT	TSOT
Book sort-title (multi-file)	Sort-album	sort_album (album-sort)	soal	ALBUMSORT	TSOA
Chapter number (multi-file)	Track number	track	trkn	TRACK	TRCK
Author	Artist	artist	©ART	ARTIST	TPE1
Author (optional override)	Album artist	album_artist	aART	ALBUMARTIST	TPE2
Narrator	Composer	composer	©wrt	COMPOSER	TCOM
Genre	Genre	genre	©gen	GENRE	TCON
Release date *	Date	date	©day	YEAR	TDRC/TYER
Description	Description	description (MP4) TIT3 (MP3)	desc	SUBTITLE	TIT3

* When being as precise as possible the ISO 8601 format of the string is yyyy-MM-ddTHH:mm:ss (in UTC), but the precision may be reduced by removing as many time indicators as wanted.
Hence valid timestamps are yyyy, yyyy-MM, yyyy-MM-dd, yyyy-MM-ddTHH, yyyy-MM-ddTHH:mm and yyyy-MM-ddTHH:mm:ss.

ADDITIONAL SPECIFICATIONS

There is a dedicated Narrator tag that Audible uses (©nrt) but since not all audiobooks come from Audible, most of the audiobook community uses the composer tag instead for tagging.

Artist/Performer is used for the author because it's the most prominent tag for a person associated with the media, with Composer being the second most commonly viewed tag.

If you want to cover your bases (i.e. have accurate metadata, but also use the community standard metadata working on popular apps) you must do it this way:

Author(s)(iTunes / ID3 -- I will try to include options for both 2.4 and 2.3)
Artist:
©ART / TPE1 (standard)
©aut or auth / TIPL: Author or IPLS: Author (OCD-version)

First Author (for multi-author books, or editor of an anthology. Whoever you want to be most recognized/searchable for this book)
aART / TPE2 (standard)

Narrator(s)
©com or ©wrt / TCOM (standard)
©nrt or perf / TMCL: Narrator or IPLS: Narrator (custom)

Publisher
There are 2-3 different tags that can be used for this. ©pub/TPUB are both the most common and the most accurate, but some apps won't recognize this. If they don't, they're probably looking for ©lab (label -- not sure if there is an ID3 equivalent) or possibly even the studio tag, which I can't remember off the top of my head.

Series
Ideally individual books in a series should be tagged in the way discs in a multi-disc album are tagged. So "Album" would be the series title, and then "Title" would be the book title, and you'd use the "disc#" tag for the series index. However there are any number of ways to do this, and none of them are going to be universal. Here are a couple, however:

Audible uses series and series-part for this purpose (I don't remember off the top of my head what the MP4 code is for these, but I think it's one that Audible created and not part of the standard library).
I think that tag probably gets lost when converting to ID3 though, so you could create TXXX: Series and TXXX: Series Index tags for this. That's one option.

Some people use the Grouping or Content Group tags for this:
©grp / TIT1 - Series title

A more precise way to go is to use the movement/movement index tags:
©mvn / MNVN - Series title
©mvi and MVIN - Series index
That seems to be the second-most popular option, probably because those tags already exist in both MP4/Quicktime tagging and ID3 tagging.

If you have books that are in a multi-series universe (eg. Terry Brooks' Shannara universe), you could also use the "Work" tags in addition to the "Movement" tags:
©wrk / TIT1 - Universe title

If you use .m4b files rather than .mp3, you can also use the tags available for TV Shows:
tvsh - series title
tven - book title
tves - series index
tvnn - Universe title
tvsn - Universe index (i.e. where the sub-series falls within the universe as a whole, so you know which subseries to read in which order.)

There's no real advantage either way. Some apps don't look for series data but they look for the subtitle so they can display "Series Title, Book Index".

Thanks in advance!