Allow hidden values anywhere in the tree

QA ticket http://manage.dimagi.com/default.asp?141304

![hidden values in tree](https://cloud.githubusercontent.com/assets/464726/4638698/19e0f4a6-53fe-11e4-9162-9aabbbda3e16.png)

This is a variation of https://github.com/dimagi/Vellum/pull/261
I was wrong about this. Will change it.