[entities] Optimize CWUser.properties.
This property gets called each time a user connects to a repository,
which means a lot.
This optimisation cuts its execution time by 5 according to my tests
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html