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After this, all ERP5 objects become instances of portal type classes

Preferences:
* all the trickery for preferences is gone and is handled by a specific
  accessor holder holding all preference methods

Property holders
* our Base.aq_portal_type property holders are not used anymore:
  the "property holder" becomes the portal type class itself and the
  set of accessor_holder classes in the mro of the portal type class:
  portal-type-specific methods are on the portal type class, while
  portal-type-independant method are put on the accessor holder ancestors
* the portal type meta class now also inherits from "PropertyHolder" to
  provide the same introspection interface and methods.
  (In the future this class / interface will need to be refined)

Bootstrap/migration:
* bootstrapping/migration from older instances: provide with code able to
  import XML from ERP5/bootstrap/ to load necessary tools from almost any
  instance state
* migrate in Bus...