Added updateItem to ManagedItemProvider and ItemRegistryChangeListener.
Added addGroupName and removeGroupName to GenericItem and returning immutable Lists now.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Fixed group member assignment in itemUpdated in ItemRegistryImpl.
Added test for group member assignment and itemUpdated in ItemRegistryImpl.
Added test for listeners in ItemRegistryImpl.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Merge branch 'groupitem-persistence' into integration