[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.

action box
    Area visible in the upper left column. You have a list of available actions on the entity. The most frequently used entry is `modify`.

object
    All element created in the application
    Example: project, ticket, user, ...

relation editing module
    HTML widget that let you define new relations amongst objects.

relation
    It's a kind of 'smart' link between 2 objets of the application. It has a specific sense that determine dynamic behaviour and add a new logic of the content.