Add support for Long Description in the GUI for mp4 tag

[milkas9]

I want to add long descriptions for my home videos so that it is picked up by Plex.

mp4's "description" field only supports up to 255 bytes while "long description" is almost unlimited.

I have successfully added the long description through the cli with
tageditor set --max-padding 429496729 --tag-pos back mp4:ldes='LONG_DESCRIPTION' -f video.mp4

I would like to do this through the GUI instead if possible.

[Martchus]

To support this in the GUI I'll just have add a mapping to a "known field". This is of course also beneficial to the CLI. It is just the question what known field to map it to and whether there's an equivalent field in other tag formats.

I can map this to "Synopsis" like ffmpeg: https://github.com/FFmpeg/FFmpeg/blob/226fcc7258ca0a6bc74c19acc8d2fe181393bfb1/libavformat/movenc.c#L4626

Hydrogen audio suggests mapping to "Long description", though: https://wiki.hydrogenaud.io/index.php?title=Tag_Mapping
This is also used by Atomic Parsley and others: https://github.com/get-iplayer/get_iplayer/wiki/tagging

Mp3tag calls it "POSTCASTDESC": https://docs.mp3tag.de/mapping/#podcastdesc

This library calls it "Episode description": https://github.com/judwhite/IdSharp/blob/13a4e86f99f32a589d2118c49477c2087c6e0305/IdSharp.Tagging/ID3v2/Classes/FrameContainer.FrameCreation.cs#L277

The mapping of ID3v2 fields TIT3 and TDES should probably be considered as well.

I guess "Long description" makes most sense. Either way, this should be a trivial change. I just have to see when I'll have the time to implement it and do a new release.

[milkas9]

Thank you.
I was previously using Atomic Parsley in my python script for adding long description, but now I have switched to tageditor with the command above as I don't want to rewrite the file if possible.

I am also currently using Subler on Mac to add long description without rewriting the file, but I am also looking for a windows app that can do the same. I figured since tageditor can already do it through CLI, adding the option to GUI shouldn't be too hard.

[Martchus]

I have added this as "Synopsis" in Martchus/tagparser@7ba8e0f because "Synopsis" is already a known field with a mapping for Matroska. So I only added mappings for MP4 and ID3v2.