Disable HTML widget buttons when readonly

issue10737
review334951002
(grafted from f7d8d964d69dc1b6a17725553e41a1924e7e0031)

--HG--
branch : 6.0

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

@@ -159,7 +159,8 @@ class HTML(Widget, TranslateMixin):
 
     def _readonly_set(self, value):
         super()._readonly_set(value)
-        self.button.set_sensitive(not value)
+        for button in self.widget.get_children():
+            button.set_sensitive(not value)
 
     def translate_dialog(self, languages):
         languages = {l['name']: l['code'] for l in languages}