Make date readonly when the number is already set

Is should not be possible to modify the invoice_date when a number is set,
otherwise it should be possible to make an invoice which is not consecutive
because we only check it when creating the number.

invoice.py

@@ -2,6 +2,7 @@
 # the full copyright notices and license terms.
 from trytond.transaction import Transaction
 from trytond.pool import Pool, PoolMeta
+from trytond.pyson import Bool, Eval
 
 __all__ = ['Invoice']
 
@@ -24,6 +25,15 @@ class Invoice:
                     'permitted, because of invoice number and date '
                     'correlation.',
                 })
+        if 'number' not in cls.invoice_date.depends:
+            new_readonly = Bool(Eval('number'))
+            readonly = cls.invoice_date.states.get('readonly')
+            if readonly:
+                new_readonly |= readonly
+            cls.invoice_date.states.update({
+                    'readonly': new_readonly,
+                    })
+            cls.invoice_date.depends.append('number')
 
     @classmethod
     def validate(cls, invoices):