[PATCH 12/17] layouts: refactor the entity_node and tree layout logic

The previous logic directly associated xtree_node_t elements with entities.
The list of nodes associated to an entity was thus directly made of xtree_node_t
elements each one pointing directly to the entity.

An entity being layout agnostic, that is to say not bound to a particular layout
it was not possible to look for the xtree_node_t element associated with a
particular layout in an entity. This was due to the fact that the entity_node
element was not used as an intermediate level to link the entities and their
different xtree_node_t elements. Indeed, this is the role of the entity_node to
associate a position in a particular layout with an entity.

The new logic ensures that when building a layout relational structure :

- (1) an entity node associated with the targeted layout is created for the
  entity and references both the layout and the entity it is associated too.
- (2) the entity node is the element that is added to the (tree) relational
  structure of the layout resulting in the creation of a relational node
  (xtree_node_t) that will reference it directly.
- (3) the created relational node (xtree_node_t) is referenced in the associated
  entity node in order to provide a way to go back to the position in the
  relational structure from the entity node, thus enabling to search the
  neighborhood of an entity for a particular layout only knowing the entity
  and the layout to look at.

The packing logic was modified to take into account the new level of indirection
(the entity_node element) when walking the relational structure of a particular
layout. The debug dump logic was modified too to reflect the changes.