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.


/reviewed-on !980
7 jobs for feat/accounting_ui_test_coding_style in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

05:29:38

failed ERP5.UnitTest-Master

07:07:41

passed SlapOS.Eggs.UnitTest-Master.Python2

01:30:00

passed SlapOS.Eggs.UnitTest-Master.Python3

00:30:41

passed ERP5.CodingStyleTest-Master

passed SlapOS.Eggs.UnitTest-Master.Python2

00:27:57

failed SlapOS.Eggs.UnitTest-Master.Python3

00:23:20