Move ftw.treeview into opengever.core

:construction: :construction_worker: 

[`ftw.treeview`](https://github.com/4teamwork/ftw.treeview) isn't being used anywhere else than in OpenGever (and actually contains OpenGever specific code). It's also a hard dependency, there's no way of running OpenGever without it.

Therefore it should be moved into `opengever.core`.