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.