[entities] Fix backward compat of IDublinCore adapter wrt dc_long_title
`entity.dc_long_title()` used to fallback to `dc_title()`, and most entity types
were relying on this, thus only implementing the later.
Since introduction of the IDublinCore adapter, if one call
`entity.dc_long_title()` on an entity that only implements `dc_title()`, it will retrieve
the adapter which will then call its own `title()` method instead of the
`dc_title()` method of the entity as expected.
Fix this by calling instead `entity.dc_title()` which will eventually kick in the
backward compat layer if necessary.