- Most property sheets are in a new 'erp5_property_sheets' business template,
  which is installed first (even before erp5_core).
- Bootstrap is done by fetching the few required objects directly from core BT.
- Properties/constraints/categories stores the reference inside the id
  (instead of 'default_reference'), so that BT is exported with nice filenames.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@43482 20353a03-c40f-0410-a6d1-a30d3c3de9de