[multi-sources] support for moving an entity from an external source (closes #343818) Original need is to move a user from a ldap source to the system source so we can delete it from ldap without loosing information into the cubicweb instance. We can't wait for the user to be deleted from the ldap since it will be too late then to get back user attributes, so it has to be a manual operation to operate before actual deletion. This makes sense for other sources as well. So the idea is to make the "Any cw_source CWSource" relation editable by managers, and to watch changes of it. We then check the move is possible (ie from an external source to the system source) and do necessary stuff (essentially changing source information and copying data into the system source). Remaining pb is that we don't want the moved entity to be reimported later. To distinguish this state, the trick is to change the associated record in the 'entities' system table with eid=-eid while leaving other fields unchanged, and to add a new record with eid=eid, source='system'. External source will then have consider case where `extid2eid` return a negative eid as 'this entity was known but has been moved, ignore it'. Notice no ui is provided yet, it has currently to be done in a c-c shell.

Javascript Coding Standards
---------------------------

(Draft, to be continued)

:Naming: camelCase, except for CONSTANTS

Indentation rules
~~~~~~~~~~~~~~~~~
- espace avant accolade ouvrante

- retour à la ligne après accolade ouvrante (éventuellement pas
  de retour à la ligne s'il y a tout sur la même ligne, mais ce n'est
  pas le cas ici.

- no tabs


Documentation
~~~~~~~~~~~~~
XXX explain comment format for documentation generation


Coding
~~~~~~
- Don't forget 'var' before variable definition, and semi-colon (';') after **each** statement.
- Check the firebug console for deprecation warnings


API usage
~~~~~~~~~
- unless intended, use jQuery('container') rather than jqNode('container')


See also
~~~~~~~~
http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml