Album.items() should sort by track number first

As pointed out by @djl in #494, the `Album.items()` method is returning tracks sorted by the global sort specification for items (the `sort_item` config option). By default, this is `artist+ album+ disc+ track+`, but this means that albums with multiple track artists appear "out of order" (since `artist` precedes `track`).

An easy way to fix this would be to make the default sort for items use `albumartist`, and not `artist`. This would have the nice effect of grouping albums correctly when searching for items—something that I believe that we had in the past. But it makes less sense for singletons, where album artists may not even be present.

Any thoughts?

Start by inspecting the Album.items() method and the sort_item configuration described in the issue. Compare the current artist-based ordering with albumartist-based ordering, including albums with multiple track artists and singletons. Done means the intended ordering is decided and Album.items() consistently produces it without breaking singleton sorting.

Written by the indexing model from the issue text.