EZP-21213: FieldType\Image must respect legacy images naming

http://jira.ez.no/browse/EZP-21213

In order to match legacy's behaviour, `FieldType\Image` must use different naming methods for draft & published images.

This PR implements this by adding events at `FieldStorage` (SPI) that can be implemented by External Storage handlers.

The `PostPublishFieldStorageEvent` is in this particular PR implemented in `ImageStorage`. It checks if the image referred to by the Field is a draft image,  and if yes, renames it to its actual final name.

## TODO
- [ ] test edge cases
- [ ] take care of tests