* Add bt5 dependencies:
  + erp5_demo_maxma_rule: Invoice*Rule rules.
* Move Documents used by several bt5 to erp5_core rather than add depends on erp5_core.
  Once migration of Products to ZODB Components will be finished, there will be moved to
  their appropriate bt5s as it would be complicated to reorganize bt5s too at this point.
  + Invoice{Line,Cell}: used by erp5_payroll.