Save playlist digest on loading.

Save playlist digest on loading.

The previous implementation felt that playlists were not saved
(.is_saved() -> False) when the playlists had not been altered since
loading.

We *don't* save the digest when we load from a string, because
that string is not necessarily on disk and we *do* want to save
it, even if the playlist hasn't change since loading.

I also removed the return bits, since FilePlaylist._from_xml_doc()
doesn't return anything.