[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.

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Schema of the data

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First take a look at the data schema_ then try to remember that you are browsing

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through a heap of data by applying stylesheets to the results of queries. This

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site is not a content management system with items placed in folders. It is an

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interface to a database which applies a view to retreived data.

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.. _schema: schema