Refactor OutlineControlsWidget, add remove button

* Adders turn out to be very instance specific, so abstracting them
  makes things pretty awkward. Rather than generate adders from a
  configuration object, mix in GroupElement and use its API for managing
  items.
* Add remove button and event to OutlineControlsWidget
* Add removable state to OutlineItemWidget
* Use IconedElement to render + icon for adders, rather than faking it
  with a button that doesn't do anything
* Blur focused inputs on page change
* Add method for getting the closest page to another, taking into
  consideration depth when a booklet is outlined

Change-Id: I35ae249b0b1597188dc6d06cdaf78ba08027be80