Exaile should sort in natural filename order in absence of track number info

[Marty]

For some reason my ogg vorbis files were created without the tracknumber tag (and I'm off to fix that now!).

I'm not sure what exaile is using to sort tracks from an album instead, but it should fall back on the natural sort order of the filename. This seems to me to be the only other place the encoder might have "stored" the track numbering when ripping the album.