[js] better support for no js in tabs (closes #2162092)

[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.

[datafeed] when checking for deletion using cwentity xml parser, use vid=xml to limit resources consumption

[repository] ldap-feed source. Closes #2086984 datafeed based source which copy a subtree of the ldap directory into the system database. Authentication still go through ldap though. Pros: * don't need temporary tables and such for multi-sources RQL queries execution * much more flexible to enhance / configure behaviour (you simply have to replace the parser) * run better when ldap isn't reachable Cons: * no more 'on the fly' discovery of users (though a user authenticating itself will be automatically added if it doesn't exist in the db yet) * synchronization may be heavy if there are a lot of users A new cw.server.ldaputils containing code in common between former ldapuser and new ldapfeed sources has been introduced. Also ldapuser source now uses url instead of custom host/protocol option so it looks like a datafeed source (could be improved).