Fix EZP-23028: eZObjectRelationList not considering the selected object

Link: https://jira.ez.no/browse/EZP-23028
## Description
When adding an item to a relation list using the searchbox, an error message was displayed. This was happening because the validation function only checked for items in database (the way they are set in browse mode) and not in `$POST`.
This patch makes the validate method accept items from the searchbox by looking into `$POST`.

## Tests
Manual tests