[fti] add cw_fti_index_rql_limit method
Improves on and deprecate cw_fti_index_rql_queries: execute the rql
directly, so that we don't need to compute the set of eids up-front, but
can just keep track of the last seen entity. Use the new method in
reindex_entities.
Keep calling cw_fti_index_rql_queries if it has been redefined (e.g in
cubes).
Related to #3621392.
The facets system
-----------------
Facets allow to restrict searches according to some user friendly criterias.
CubicWeb has a builtin `facet`_ system to define restrictions `filters`_ really
as easily as possible.
Here is an exemple of the facets rendering picked from our
http://www.cubicweb.org web site:
.. image:: ../../images/facet_overview.png
Facets will appear on each page presenting more than one entity that may be
filtered according to some known criteria.
Base classes for facets
~~~~~~~~~~~~~~~~~~~~~~~
.. automodule:: cubicweb.web.facet
.. _facet: http://en.wikipedia.org/wiki/Faceted_browser
.. _filters: http://www.cubicweb.org/blogentry/154152