Make AttachedFile immutable. This is consistent with SimpleTag and
Chapter and avoids using attached files which do not have all required
attributes.
Provide methods to insert and remove a single simple tag, so that
they can be modified without setting all of them while still not
exposing internal lists to the API.
Use DATE_RECORDED instead of DATE_RELEASED for year() and the "DATE"
property. This is more consistent with other tag formats, e.g. for ID3v2
"TDRC" is used, which is the recording time.
