[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.

CSS Coding Standards
--------------------

(Draft, to be continued)

:Naming: camelCase

Indentation rules
~~~~~~~~~~~~~~~~~
- 2 espaces avant les propriétés

- pas d'espace avant les ":", un espace après

- 1 seul espace entre les différentes valeurs pour une même propriété


Documentation
~~~~~~~~~~~~~
Please keep rules semantically linked grouped together, with a comment about
what they are for.

Recommendation
~~~~~~~~~~~~~~
- Try to use existing classes rather than introduce new ones

- Keep things as simple as possible while in the framework

- Think about later customization by application

- Avoid introducing a new CSS file for a few lines of CSS, at least while the
  framework doesn't include packing functionalities