[IMP] fields: extend selection with a given order

When extended with the selection_add parameter, a singleton (value,)
where `value` appears in the overriden selection, can be used.
The new values are inserted in an order that is consistent with the
overridden selection and this list.

selection = [('a', 'A'), ('b', 'B')]
selection_add = [('c', 'C'), ('b',)]
> result = [('a', 'A'), ('c', 'C'), ('b', 'B')]