Add content library

This set of changes implements the content library. Usage is quite simple:

* The site administrator must add a new `library` item in the euphorie section of `euphorie.ini` with the path (relative to the site root) for the survey that is used by the library.
* A new *Library* button is added to the add-bar in the CMS. Clicking this opens a new library page.
* The library page shows the complete contents of the library (essentially a sitemap of the survey). All items that can not be added to the current location due to content type or depth limitations are greyed out.
* Clicking the *Insert* button for an item copies it into the current survey, making sure to give all new items a new object id.

The current implementation is deliberately minimal since we expect this to be reworked later as part of a new UI design. Likely changes for a full version are:

* show the library as a popup instead of a new page
* allow searching through the library
* allow multiple surveys to be part of the library
* allow sector organisations to offer their own surveys as part of the library