Playlist duration may return an erroneous value

In case a playlist contains a media that has no duration, the playlist duration may be wrong.

Example:

In this case toto and tutu have the same medias but toto has an additional stream (with an unknwon duration). With the current API, the ports cannot detect this kind of error without querying the playlist medias.

It would be great to have another field saying if the playlist contains only medias with duration or not.