Formatting changes on ufront.db.Object
Get rid of `pop` and `push`, they're not useful enough to justify the extra complications
Add `saved` signal to Object
Fix for unserialize on server recreating bList from IDs correctly
Better handling of unsaved joins:

- Unsaved joins happen when `ManyToMany.add()` is called on the client
- Or on the server, when one of the 2 joint objects does not have a valid ID.
- Now we keep track of objects which are not saved. If we serialize, they are included, not just the ID.
- When an unsaved object is added to a join, we wait for it's saved signal, and then create the join.
- Our unit tests now pass, at least on first compilation...