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

http://jira.ez.no/browse/EZP-21213

*Requires https://github.com/ezsystems/ezpublish-kernel/pull/632* (FieldStorage events implementation).

In order to match legacy's behaviour, `FieldType\Image` must use different naming methods for draft & published images.

A`PostPublishFieldStorageEvent` is 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
- [x] test edge cases
- [x] Changing the image on later versions don't seem  to work at all
- [x] take care of tests
- [x] Manage the fact that API `FieldType\BaseIntegrationTest` creates test content without a Location, making it problematic to test the expected name, since it depends  on the location's path_identification_string
- [x] Refactor to use the newly added `FieldStorage\EventAware` in #632