Album dirty tracking should be less aggressive

[sampsyo]

As pointed out in #853, recent refactoring has broken a behavior in album-level metadata modifications. If, for example, an album has field foo equal to bar, then beet mod foo=bar is a no-op due to dirty tracking. This is problematic when the tracks have inconsistent data—it would be good for this change to propagate to the tracks even when no change has made.

This means we need to change the way dirty tracking works for the Album class to consider a field dirty on assignment, even when the new value equals the old value.