New Toolbar API

This is a work in progress, tests are failing and some stuff is still missing.

Massively refactored the toolbar (Python) API.

Things that still need to be done:

* [x] Finish documentation
* [x] Fix broken tests
* [x] Add a ``find_item`` API and allow items to be added relative to other items.
* [x] Talk with @FinalAngel what needs to be done in the HTML/JS bridge. Mostly I want to change the class name of the build/edit mode switcher to something more explicit.
* [x] Find a solution for ``get_menu``
* [x] Find a solution for ``close_on_url_change``.