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.

bt5/erp5_project/PortalTypeTemplateItem/portal_types/Project%20Cell.xml

@@ -60,7 +60,7 @@
         </item>
         <item>
             <key> <string>type_class</string> </key>
-            <value> <string>BudgetCell</string> </value>
+            <value> <string>OrderCell</string> </value>
         </item>
       </dictionary>
     </pickle>