Bug 841977 - Show track number instead of index. Also disc number if relevant.

Includes dependencies that have to land before this. ie will be rebased when time comes.
Could you do this outside of the `sort` function? You'll end up calling this way more often than you need to. I'd probably be ok with just sorting and then checking the disccount/discnum of the *last* element of the array, although that's not exactly the same as what you're doing here, since it wouldn't check all the tracks for the largest disccount.

That said, I'm *also* ok with always showing the disc number if it's present.