privacy: disconnect GSettings changed signal properly

This is a race condition between the destroy handlers I think. It's possible for the changed signal to be emitted after the label is destroyed, and its cb calls gtk_label_set_text().

Reproducable by switching rapidly between the Privacy panel and another one. I was using my touch screen because it's possible to do it faster that way and that makes the bug easier to trigger, but you can do it with the mouse. (That's how I found this initially.)

(The second commit is a bonus commit to not leak some strings.)