Behaviour of legacy composition is also slightly changed:
During composition of a delivery line, a model line on the delivery now has
priority over all specialise values reachable from the delivery line.

git-svn-id: https://svn.erp5.org/repos/public/erp5/sandbox/amount_generator@39544 20353a03-c40f-0410-a6d1-a30d3c3de9de