Decorate post method of sale point with the button decorator

Closes #14594
(grafted from 3c30bf919e36e7c8f6b68666ae9508f7a58b0f74)

--HG--
branch : 7.0

modules/sale_point/sale.py

@@ -328,6 +328,7 @@ class POSSale(Workflow, ModelSQL, ModelView, TaxableMixin):
         cls.save(sales)
 
     @classmethod
+    @ModelView.button
     @Workflow.transition('posted')
     def post(cls, sales):
         pool = Pool()