[BUGFIX] Tx_Flux_Form_Container_Object should extend Tx_Flux_Form_AbstractFormContainer

This has a implication for l18n. If ``Tx_Flux_Form_Container_Object`` were to extend ``Tx_Flux_Form_Container_Container`` then the ``name`` argument would transform to:

```xml
flux.ID.containers.ObjectID
```

but it should actually transform to:

```xml
flux.ID.objects.ObjectID
```

as per ``Tx_Flux_Form_AbstractFormComponent::getLabel()``