Silva-core-interfaces

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* More API has been update to match pep8 recommandation.

* Improve and add more Silva events.

* Improve and add more Silva exceptions.

* Add a new base, ``VersionedNonPublishable``, for versioned content
that are not publishable.

* More API have been improved on images, titled object and security objects.

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* Major API refactoring on Silva contents.

* Add generic exceptions.

* Improve events.

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* Update interface documentation.

* Remove no longer used interfaces.

* Update interface inheritance tree to match content one.

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* Subscription interfaces moved to silva.app.subscriptions.

* Authorization management API have been updated.

* Content interfaces have been updated and documentation improved.

* Events have been added for publication actions.

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* Remove unused interfaces,

* Add missing interfaces for contents.

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* Clean some interface that where including ``self``.