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New features:
- Added ``IBlockFieldDeserializationTransformer`` and its counterpart,
``IBlockFieldSerializationTransformer`` concepts, use subscribers to
convert/adjust value of blocks on serialization/deserialization, this enables
an extensible mechanism to transform block values when saving content.
Added an html block deserializer transformer, it will clean the
content of the "html" block according to portal_transform x-html-safe settings.
Added an image block deserializer transformer, it will use resolveuid mechanism
to transform the url field to a UID of content.
Move the resolveuid code from the dexterity field deserializer to a dedicated
block converter adapter, using the above mechanism.
[tiberiuichim] (915)