Prevent changing the company when party is set

As many fields are set using party which depends on the company context, the
company can not be changed once they are set.

issue10289
review332591002

invoice.py

@@ -50,8 +50,11 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
     _depends = ['state']
 
     company = fields.Many2One(
-        'company.company', "Company", required=True, select=True,
-        states=_states, depends=_depends)
+        'company.company', 'Company', required=True, select=True,
+        states={
+            'readonly': _states['readonly'] | Eval('party', True),
+            },
+        depends=_depends + ['party'])
     company_party = fields.Function(
         fields.Many2One(
             'party.party', "Company Party",