DMM: Use a special 'nepomuk' graph to store certain properties

Each application contains its own graph. Similarly, there exists one
graph owned by 'nepomuk', which contains the following properties -

* nao:lastModified
* nao:created
* nie:url

This simplifies removal of resources, as these properties should ideally
never be removed. Also, they are technically managed by nepomuk and not
some other application.

Changed all the tests to reflect this change.

We also no longer check for trailing graphs as they might always be some
trailing graphs of some applications.