remove count() calls

I removed this method in an earlier commit.
revamp ResultIterator (now Results)

This new design lets us provide better common functionality. The `get()`
method is a particularly good example: we can now avoid the try/except dance
that was previously necessary to get the only result for a single-result
query. This also lets us continue using `len()` on the result of
`album.items()`, which previously returned a materialized list.
album.store() now also affects tracks
add load() method to LibModel
fix dirtying when unchanged, artpath wrapping

Also some naming mistakes.
fix outdated SQL query