Loaded attributes and default sorting management

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* The class attribute `fetch_attrs` allows to defined in an entity class

  a list of names of attributes or relations that should be automatically

  loaded when we recover the entities of this type. In the case of relations,

  we are limited to *subject of cardinality `?` or `1`* relations.

* The class method `fetch_order(attr, var)` expects an attribute (or relation)

  name as a parameter and a variable name, and it should return a string

  to use in the requirement `ORDER BY` of an RQL query to automatically

  sort the list of entities of such type according to this attribute, or

  `None` if we do not want to sort on the attribute given in the parameter.

  By default, the entities are sorted according to their creation date.

* The class method `fetch_unrelated_order(attr, var)` is similar to the

  method `fetch_order` except that it is essentially used to control

  the sorting of drop-down lists enabling relations creation in

  the editing view of an entity.

The function `fetch_config(fetchattrs, mainattr=None)` simplifies the

definition of the attributes to load and the sorting by returning a

list of attributes to pre-load (considering automatically the attributes

of `AnyEntity`) and a sorting function based on the main attribute

(the second parameter if specified otherwisethe first attribute from

the list `fetchattrs`).

This function is defined in `cubicweb.entities`.

For example: ::

  class Transition(AnyEntity):

    """..."""

    id = 'Transition'

    fetch_attrs, fetch_order = fetch_config(['name'])

Indicates that for the entity type "Transition", you have to pre-load

the attribute `name` and sort by default on this attribute.