Silva-core-smi

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* Various improvements on buttons.

* Add support for error pages in SMI.

* Update to use ``silva.core.interfaces`` and directly ``five.grok``.

* SMI components are register on a SMILayer.

* The editor buttons (kupu editor, forms editor) can be disabled in
IDocument types that don't support a particular editor
(e.g. SilvaNews News Items don't support the forms-based editor)

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* Fixed editor buttons on publised documents.

* Keep lookup button selected on lookup screen.

* Remove deprecation warnings.

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* Initial release