Better management of readonly on Group

There are some cases where readonly must be always True, otherwise we must rely
on the value set.

tryton/gui/window/view_form/model/field.py

@@ -742,8 +742,8 @@ class O2MField(CharField):
     def state_set(self, record, states=('readonly', 'required', 'invisible')):
         self._set_default_value(record)
         super(O2MField, self).state_set(record, states=states)
-        if self.get_state_attrs(record).get('readonly', False):
-            record.value[self.name].readonly = True
+        record.value[self.name].readonly = self.get_state_attrs(record).get(
+            'readonly', False)
 
     def get_removed_ids(self, record):
         return [x.id for x in record.value[self.name].record_removed]

tryton/gui/window/view_form/model/group.py

@@ -35,17 +35,23 @@ class Group(SignalEvent, list):
         self.load(ids)
         self.record_deleted, self.record_removed = [], []
         self.on_write = set()
-        if self._context.get('_datetime'):
-            self.readonly = True
-        elif not MODELACCESS[self.model_name]['write']:
-            self.readonly = True
-        else:
-            self.readonly = readonly
+        self.__readonly = None
         self.__id2record = {}
         self.__field_childs = None
         self.exclude_field = None
         self.pool = WeakSet()
 
+    @property
+    def readonly(self):
+        if (self._context.get('_datetime')
+                or not MODELACCESS[self.model_name]['write']):
+            return True
+        return self.__readonly
+
+    @readonly.setter
+    def readonly(self, value):
+        self.__readonly = value
+
     @property
     def domain(self):
         if self.parent and self.child_name: