Add BlockReference to reference structured part of the content of a document or an object property.

A BlockReference is a reference to a structured part of the content of a document or an object property. With the additional provision that the meaning of block references depends on their usage, may not be unique and are not necessarily a way to reach the referenced instance.

We may have different kind of block references, for different purposes (for example identifying a header in the content, linking signature to macro block, etc...). This reference will be added into xwiki-platform-model, since it is general purpose, and since it is linked with the an avoidable change in model to add the new EntityType.BLOCK.

A BlockReferenceResolver may resolve a reference from the referenced instance or another representation into a validated BlockReference object. It will be also defined in xwiki-platform-model without any ParametrizedType helper, and no default implementation will be provided, since the meaning of a block reference is not application wide.

Related discussion: http://xwiki.markmail.org/thread/5fkcvulshnzihqgc