Silva-core-layout

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* Improve legacy stability, specially error pages on non-Silva content.
Add tests.

* Add a content provider to manage the favicon.

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* Add a ``lang`` attribute on the HTML tag of the Porto layout.

* Apply ``silva.core.layout`` layout on all Silva content.

* Update tests.

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* SMI Maker interface moved to `silva.core.smi`.

* A new content provider ``headinsert`` to extends pages heads.

* Default Porto template have been improved.

* Add JQuery and JQueryUI resources. You can use them in your layout
by inheriting from ``silva.core.layout.jquery.interfaces.IJQueryResources`` or
``silva.core.layout.jquery.interfaces.IJQueryUIResources``.

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* Replace ``z3c.form`` with ``zeam.form``.

* Improve default Porto layout.

* Add a ``silva.core.layout`` legacy layout to replace the one of
SilvaLayout, with support for overrides.

* Move some dependent code from SilvaLayout to ``silva.core.layout``:
traverser to set skin, ``IMetadata`` adapter, skin metadata set.

* Skin is now only set while traversing to the publish object. This
prevent to get it changed to something else while accessing other
that might have a different skin.

* SMILayer moved to ``silva.core.smi``.

* Update code to be compliant with Zope 2.12 and Python 2.6.

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* Porto skin is now not registered by default but only usuable as base
skin for more complicated skins.

* Add support to render error pages in modern skins.

* Various bug fixes and improvement in the Porto skin:

* Set HTTP cache header by default.

* Improve navigation generation code.

* Use silva.core.interfaces and silva.translations.

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* Improve Porto skin to be extended more easily.