Rating on non-BFS

[m199]

I tried writing (and reading) the POPM frame to save the ratings in files that are not stored on BFS. I started with the Windows Media Player 9 identifier in the email field, with a conversion of 1-255, but we need to test whether this is the end result. Actually, it's so old that all current players should support it and it seems the way to go here. https://docs.kde.org/trunk_kf5/en/kid3/kid3/settings-menu.html

[m199]

The rating in the metadata is working (more or less) and be read from various players.
How should we handle read-only storage media: should we refuse to assign a rating and simply report it, leave it in the cache (until it is updated, at which point it would be gone), or create a second index that retains the data until the storage medium becomes writable?