Move ID3Metadata functionality to display_metadata method

The problem with ID3Metadata is that it changes the behavior of
Metadata methods in undesirable ways (see PR #273).

This adds a display_metadata method to File classes which accepts a
metadata object, and possibly returns a copy with the values modified
as they'd be displayed/saved. This should more composable and less
error-prone than overriding Metadata methods across the board.