Collective.documentgenerator

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- Handle case of rendering value of single selection widget.
[sdelcourt]

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- In `DocumentGenerationView._render_document`, pass `portal` as `imageResolver`
to `appy.pod.renderer.Renderer` so private images can be accessed by
LibreOffice in XHTML fields.
[gbastien]

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- Pass `**kwargs` to DocumentGenerationView._render_document so it is possible to pass
arbitrary parameters to appy.pod.renderer.Renderer that is called in _render_document
and to which we also pass the `**kwargs`.
This way, it is possible for example to turn `Renderer.raiseOnError` to True.
[gbastien]
- Added meta_type for content_types `PODTemplate`, `ConfigurablePODTemplate`, `SubTemplate`
and `StyleTemplate`, this way it can be used to filter out objectValues/objectIds.
[gbastien]
- Added a validator on the configurablePODTemplates which check if the chosen generations
formats are corrects with the kind of file provided.
[boulch, DieKatze]

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- CSS fix, display POD templates in the viewlet using display: inline-block;
instead of display: inline; so attached tags may be aligned on it.
[gbastien]
- Added 'description' to the list of available data to display in the generationlinks viewlet.
The POD template description is now displayed when hovering the POD template title.
[gbastien]

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- Added `ConfigurablePODTemplateCondition._extra_expr_ctx` method so it is easy
to extend the context of the ITALCondition expression without overriding
the `evaluate` method.
[gbastien]

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- Make sure to not query a `None` to ensure compatibility with ZCatalog 3.
[gbastien]
- Take into account the `oo_port` paramater defined in the registry.
[gbastien]