unified way of "modernizing" files

[supermihi]

File formats such as FLAC, APE, MP3 might contain several tags (ID3v1, v2, Xiph, APE), one of which is usually preferred for that format. The current implementation of the dict-style properties-interface uses the most preferred one that exists in the file, but does not allow to switch from e.g. ID3v1 to APE on request.
If a file is using such a more or less deprecated tag format, the map exported by properties() should contain some flag indicating this, and there should be a possibility to "modernize" the file, which is completely abstract from the actual file format (similar to unsupportedData() / removeUnsupportedProperties()).