Add 'Record Label' field mapped to Publisher metadata

[emphor]

Summary

Adds support for a Record Label field throughout Mixxx, mapped to the standard Publisher metadata tag.

Changes

• Adds a Record Label field to Track Properties.
• Adds a Record Label column to the library.
• Reads Publisher metadata from supported audio files.
• Writes Record Label back to the Publisher tag.
• Stores the value in the Mixxx library database.

Testing

• Imported files containing Publisher tags.
• Edited Record Label in Track Properties.
• Verified persistence after restarting Mixxx.
• Verified tags written using Mp3tag.
• Confirmed existing Publisher tags are imported correctly.

[cr7pt0gr4ph7]

The code itself looks good, see below for further comments.

[cr7pt0gr4ph7]

I can see the value of being able to display and manipulate the Publisher tag, and think of certain DJ workflows (esp. related to electronic music) where this would be beneficial. My only concern is that the Track Info Dialog is getting more and more crowded, and is already reaching the limit of the screen height on my laptop, even without this change.

Though this usability problem could be solved by being able to dynamically show or hide fields in the Track Info dialog. This may require some trickery to make Qt layouts play nice with this, though. I may look into doing a proof-of-concept of that.

[ronso0]

My only concern is that the Track Info Dialog is getting more and more crowded, and is already reaching the limit of the screen height on my laptop, even without this change.

yeah, this will be an issue for some users.
Note that the I have #15073 to address this in a rather 'traditional way' for small screens (768px and smaller).

I suggest to discuss the design first, before you spend time on a POC