Silva-core-conf

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* ...

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* Add support for new icon API.

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* Update default factories to restrict the call to the name picker
depending on the API implemented by the content being built.

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* Remove grokker for Silva XML import and export.

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* Use ``ISilvaNameChooser`` instead of ``mange.Id`` in order to
validate the identifier of newly added items. This makes possible to
have a pluggable behavior when validating identifier for new
content.

* Update crop coordinates field to support empty coordinates.

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* Add an HTML schema field.

* Add a custom vocabulary term that support an extra field for an icon.

* Support ``VersionedObject`` content and ``ContentCreated`` event.

* Support ``silva-settings`` metadata set.

* Add a new permission ``Manage Silva settings``.

* System extension can have installers.

* Fix some bugs.