Silva-core-views

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* ...

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* Add support for customization of the HTTP headers on specific
content via ``IHTTPHeadersSettings``.

* Add an ``Last-Modified`` header on Silva content.

* Add an ``X-Powered-By`` header on Silva content.

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* Update ``IVirtualSite`` to include methods to return the site top
level path or URL (that can be different from the root).

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* Ensure in HTTP headers that if the Content-type of the reponse is
``text/html`` an charset is provided. This prevents bugs in WebOb.

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* Add some default customization marker to hide content from the
breadcrumbs, or navigation.

* Review and improve ``absoluteURL`` adapters not to rely on the Zope
2 implementation. Monkey patch the implementation into Zope 2, so it
works for ZCatalog brains too.

* Review and improve HTTP headers support.

* Review and improve ``IVirtualSite``.

* Move in the ``ILanguageProvider`` adapter from Silva.

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* Fix various bugs on ``IVirtualSite``

* Now base the layout on the generic ``grokcore.layout`` package
instead of ``infrae.layout``.

* Review support for preview on a ``View``.

* Fix various bugs in breadcrumb and preview URL.