make moving things within a chapter in toc save properly

https://www.pivotaltracker.com/story/show/56706694

this is a hack. the problem is that add/remove/change events are translated to tree:add/tree:remove/tree:change events when they are bubbled up to the parent. so currently to detect changes in toplevel nodes the opf file must listen to add/remove/change and to catch changes from further down you have to listen to the `tree:` ones too