QmitkDataStorageTreeModel now has two internal Add/Remove functions to update itself

The external accessible AddNode/RemoveNode function now registers/unregisters
possible existing observers. Afterwards the internal ones are called to update the treemodel.

The treemodel itself just calls the internal ones if the datamanager's preferences have
changed or the helper object property's value has has changed.

All oberserver are now unregistered/removed correctly