album.move() now also stores

This lets items see any modifications to the album (when the album's fields
are dirty). It's also symmetric with the same method on items.
store albums after moving art
fix NumericQuery constructor
tests: unknown fields silently match nothing

Previously, an unknown field would be ignored. Now, they're treated as
flexattrs that just haven't been set yet (and thus always hold None).
fix uses of item.dirty and item.record
a few more necessary album.store()s
don't reset mtime on awakening from DB
remove Library.{move,store} methods

These methods are now provided by LibModel, which makes dealing with items and
albums symmetric.