This container has several design goals:
- Represent all common and relevant metadata fields of audio files in a
unified way.
- Allow perfect recreation of any metadata format from the in-memory
structure. This requires that we allow non-detected fields to reside
in an "untyped" miscellaneous collection.
Like with pictures, plugins are free to store their metadata into the
m_metadata field whenever they read it. It is recommended that this
happens on loader creation; however failing to read metadata should not
cause an error in the plugin.
