#128038 Use content_field_instance_collapse() to send form values in the Content Copy export to be sure we get the original field values for checkboxes instead of the true/false values we will get otherwise.

+ a few code style fixes

#168526 followup : import/export display settings - fix typo I introduced when committing the patch.

- updated some invalid strings for translation
- updated .pot file

(+ remove test api hook, since we don't ship with testfile anymore)