Warn users of closure

When the Briar GTK window is closed, everything around Briar is shutdown. It's equal to signing out off Briar Android. Users might not be aware of this and its consequences, i.e. no more receiving nor sending of messages.

Therefore, we should show a warning dialog (using libhandy, if possible), asking the user whether they really want to sign out/close the window.

On Briar Android, nothing is shown on sign out. Maybe we should therefore show the dialog only on the first closure.