Use BlockDataGridFieldObject for rows in a BlockDataGridField

This was probably a copy paste error from the "normal" DataGridField.
Without this the BlockDataGridFieldObject class does not make any sense
and templating it is not possible.
Filter out any auto append or template rows in updateWidgets

These are not "real" elements of the MultiWidget and confuse
the updateWidgets method if len(self.widgets) changes.
This is relevant for nested datagridfields.