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* ERP5Type.PropertySheet becomes a dynamic module that returns strings
  instead of classes for compatibility:
  - the document classes can now use strings in property_sheets attribute
  - if a document uses a string instead of a class reference to point to a
    property sheet, it means that the property sheet can either be an old
    (filesytem, local, in a product) property sheet, or a zodb propertysheet

  For now, the contents of ERP5Type.PropertySheet (strings) are overwritten
  if a local PropertySheet is loaded from the disk. But with time, as
  property sheets will migrate to ZODB, the module will empty itself and
  contain only strings.

* Move all property sheets from all products to ERP5PropertySheetLegacy
  product. Only property sheets that were left 'as is' are CMF* property
  sheets.

This commit is mostly a move to another Product, and should have small
consequences on tests/existing instances.

However, if you used to import PropertySheets ...