[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.

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