API changes in cubicweb 3.14

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* `Entity.fetch_order` and `Entity.fetch_unrelated_order` class methods have been

  replaced by `Entity.cw_fetch_order` and `Entity.cw_fetch_unrelated_order` with

  a different prototype:

  - instead of taking (attr, var) as two string argument, they now take (select,

    attr, var) where select is the rql syntax tree beinx constructed and var the

    variable *node*.

  - instead of returning some string to be inserted in the ORDERBY clause, it has

    to modify the syntax tree

  Backward compat is kept with proper warning, BESIDE cases below:

  - custom order method return **something else the a variable name with or

    without the sorting order** (e.g. cases where you sort on the value of a

    registered procedure as it was done in the tracker for instance). In such

    case, an error is logged telling that this sorting is ignored until API

    upgrade.

  - client code use direct access to one of those methods on an entity (no code

    known to do that)