Use readonly screen to display Many2Many widget

The Many2Many widget should not allow the user to edit the records.
The One2Many widget should be used instead.

issue9574
review290891002

tryton/gui/window/view_form/view/form_gtk/many2many.py

@@ -91,6 +91,7 @@ class Many2Many(Widget):
             mode=['tree'], views_preload=attrs.get('views', {}),
             order=attrs.get('order'),
             row_activate=self._on_activate,
+            readonly=True,
             limit=None)
         self.screen.signal_connect(self, 'record-message', self._sig_label)