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)