Adapt privilege interface of the core layer to the needs of the privilege dialog.

[Tobias Schlauch]

The current interface in datafinder.core.item.privileges is quite general. To simplfy the implementation of the privilege dialog it would be usful to add some additional functionalties. There should be additional methods which allow to set simplified privileges (full, read-only, None) for specfic aspects of the managed files/collections (namely content-, property, and administration-specific). This would directly correspond to the forseen privilege definition use cases.