NEW: Added a callback before writing the record for the first time

This allows a DataObject to execute further code that depends on the current GridField-State, example. Set a ID to the value of the parent record:
```php
public function onBulkImageUpload(GridField $gridField)
{
    $this->setField("ParentID", $gridField->getForm()->getRecord()->getField("ID"));
}
```

Best regards,
s-m