tracklist: Provide some interface to also retrieve the last track's length if a file can be found.

[leoschwarz]

What should happen if the file is not found:

Return an error or just a None variant? Maybe this would collide with implementing better Error handling with a custom Enum of all possible error variants.

[leoschwarz]

This would be useful for stuff like generating tracklists for musicbrainz and other databases including the duration of every song, however would also require reading the metadata of the associated files and go a bit beyond the scope of this specific crate.