Proposal: Idempotency and Undo functionality

[daiplusplus]

When destructively modifying tags, store the original values elsewhere so that the original tags can be restored.

[daiplusplus]

I've added undo functionality by storing original tag values in a PRIV ID3v2 tag for MP3 files, but I can't see how to do this for FLAC files.

I haven't implement idempotency yet, though.

[daiplusplus]

For non-MP3 files, the Undo information could be stored in a text file in the same directory. This could also mean that we can ditch the JSON library dependency too if I use that for MP3 files instead of using a private ID3v2 frame.

[daiplusplus]

I've now implemented JSON-file based recovery data saving for non-MP3 files. No progress on idempotency though.