again what learned

 setRetainInstance(true) on Fragments **ONLY** retains the instance on
configuration changes. When the fragment got destroyed because there was a
shortage of memory it will be recreated through the default constructor (so no
clean way to keep instance vars).

 - setArguments(Bundle) can be one way, but does not allow to pass object
   references (for obvious reasons)
 - using getActivity() inside the Fragment binds the fragment's implementation
   to its governing activity, but seems to be the only way to get a reference to
   the data model