[FIX] ir_qweb: translatable fields

This revision is related to 5257721b

`record._columns` doesn't contain all record fields,
computed non-stored fields are not included in `_columns`,

Therefore, when a computed non-stored field was used in the QWeb xml,
this crashed with a Keyerror exception.

Using `_fields` solves this issue. Nevertheless, translate
is not always set, but is set when the fields is indeed translatable,
so the use of `getattr` was required.

opw-660072