Add default focus method on Form Widget

The editable tree is calling this method to set the focus on the edited widget
so it must be part of the API of Widget.

issue7661
review54321002

src/view/form.js

@@ -1115,7 +1115,10 @@ function eval_pyson(value){
             } else {
                 this.el.show();
             }
-        }
+        },
+        focus: function() {
+            this.el.focus();
+        },
     });
 
     Sao.View.Form.TranslateDialog = Sao.class_(Object,  {