Permit to edit amount for manual payment in processing

Payment processed manually can have mistakes in the amount.
So by going back to processing, the user can edit the amount.

issue11176
review387791003

payment.py

@@ -266,6 +266,7 @@ class Payment(metaclass=PoolMeta):
         return move
 
     @classmethod
+    @ModelView.button
     @Workflow.transition('processing')
     @cancel_clearing_move
     def proceed(cls, payments):