Silva-core-references

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- ...

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- Display the list of broken references in SMI as well.

- Improve a little the performances.

- Fix the reference lookup widget that could display an invalid edit
link to the original content in complicated virtual host setup.

- Update icons.

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- Fix bugs on service template.

- Fix scope of the CSS defined for the reference widget.

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- Optimize path computation helpers.

- Update widget's javascript to be compatible with jQuery 1.9.

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- Add a widget for ``zeam.form`` and multiple references.

- Update Formulator to match new Silva XML import / export feature.

- Introduce ``IReferable`` interface that must implemented for all
content that can be the target of a reference. This fixes problems
where cleanup wasn't called if the target wasn't a Silva content.

- ``ReferenceSet`` object have been improved performance wise.

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- Small bug fixes when the widget is not used in the SMI.