Allow it to change color and label of STEMs

[JoergAtGithub]

Feature Description

Currently we read the JSON with this information from the STEM file only. It would be nice to add write capabilities to the STEM file soundsource and editing capabilities to WStemLabel as for the track title.
The difficult part is here, that the size of the JSON inside the M4A container will change, if the length of the label text changes.

[acolombier]

If anyone is interested in looking into this, I did publish a library reusing taglib private implementation that allows to save a STEM MP4 box.

[JoergAtGithub]

Great! If I see it right, this library does generic MP4 editing operations and is not specific to the STEM format? Did you consider to open a PR at the taglib project to add this functionality there?

[acolombier]

This is on my list of things to do, but since taglib has already these implementation as private API, I thought I would need to work hard to make my case to them (stability, consistency and testing) and I haven't find the time and motivation yet.