Do not allow to mutate dict value

A copy of the value is always returned so if it is modified it does not modify
the internal value of the record.
So to update Dict value, a new dictionary must be set on the field.

Closes #13374

sao/src/model.js

@@ -2906,12 +2906,11 @@
             Sao.field.Dict._super.set.call(this, record, value);
         },
         get: function(record) {
-            return (Sao.field.Dict._super.get.call(this, record) ||
-                    this._default);
+            return jQuery.extend(
+                {}, Sao.field.Dict._super.get.call(this, record));
         },
         get_client: function(record) {
-            return (Sao.field.Dict._super.get_client.call(this, record) ||
-                    this._default);
+            return Sao.field.Dict._super.get_client.call(this, record);
         },
         validation_domains: function(record, pre_validate) {
             return this.get_domains(record, pre_validate)[0];

sao/src/view/form.js

@@ -5056,6 +5056,7 @@ function eval_pyson(value){
                     for (const key of new_names) {
                         value[key] = null;
                     }
+                    this.field.set_client(this.record, value);
                     this._display().then(() => {
                         this.fields[new_names[0]].input.focus();
                     });

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

@@ -1178,10 +1178,10 @@ class DictField(Field):
         self.keys = {}
 
     def get(self, record):
-        return super(DictField, self).get(record) or self._default
+        return (super().get(record) or self._default).copy()
 
     def get_client(self, record):
-        return super(DictField, self).get_client(record) or self._default
+        return super().get_client(record)
 
     def validation_domains(self, record, pre_validate=None):
         screen_domain, attr_domain = self.domains_get(record, pre_validate)

tryton/tryton/gui/window/view_form/view/form_gtk/dictionary.py

@@ -513,6 +513,7 @@ class DictWidget(Widget):
         self.send_modified()
         value = self.field.get_client(self.record)
         value.update({k: None for k in new_keys})
+        self.field.set_client(self.record, value)
         self.display()
 
         # Use idle add because it can be called from the callback