project: make Project Cell use OrderCell class

erp5_project does not depend on erp5_budget, so instanciating project
cell fail unless erp5_budget is installed.

Project Cell are not used by default, so it's still a bit unclear, but
with erp5_budget we can directly define budgets by project / project
lines in a more flexible (but also more complex).

Project Line uses OrderLine, so it's consistent to use OrderCell for
Project Cell.