Set group parent from previous screen only if new group does not have one

xxx2Many widgets could set a group with a different parent then the previous
one of the screen. This group should keep its parent because it comes from the
value of the field.

issue5672
review21121002

src/screen.js

@@ -863,7 +863,7 @@
                 group.on_write = group.on_write.filter(function(e, i, a) {
                     return i == a.indexOf(e);
                 });
-                if (this.group.parent) {
+                if (this.group.parent && !group.parent) {
                     group.parent = this.group.parent;
                 }
             }