[vreg] move base registry implementation to logilab.common. Closes #1916014 A new logilab.common.registry module has been created with content from * cw.vreg (the whole things that was in there) * cw.appobject (base selectors and all). In the process, we've done some renaming: * former selector functions are now known as "predicate", though you still use predicates to build an object'selector * hence `objectify_selector` decorator is now `objectify_predicate` * the top level registry is now `RegistryStore` (was `VRegistry`) Also there is no more need for the @lltrace decorator. On the CubicWeb side, the `selectors` module has been renamed to `predicates`. There should be full backward compat with proper deprecation warnings.

Startup views
-------------

Startup views are views requiring no context, from which you usually start
browsing (for instance the index page). The usual selectors are
:class:`~cubicweb.predicates.none_rset` or :class:`~logilab.common.registry.yes`.

You'll find here a description of startup views provided by the framework.

.. automodule:: cubicweb.web.views.startup


Other startup views:

*schema*
    A view dedicated to the display of the schema of the instance

.. XXX to be continued