Feature/issue 114 deep position and bonus

Fixes #114: Bonus slots, and positioning after other items.

Positions are a lot richer now: *pages* and *pageGroups* can be bonus (fully implemented in UI too), and you can position items *after* some other named position. This works recursively and repetitively, which opens the way for conditionals: we can define two questions that come after a given named question, both are included by default, but they can be skipped depending on a conditional. So only one of the two ever appears, which is what we want.

There's a new bug I hadn't noticed since introducing Jackson: #179, nearly solved.