title: Seemingly nothing happens when attempting to add to a `gtk::ListBox` from within an event handler in GTK-RS application
body: <p>I'm attempting to to add to a <code>gtk::ListBox</code> container from within the event handling closure of an unrelated widget. The list box in question is fetched via a <code>gtk::Builder</code> like so:</p>
  
  <pre><code>let notes_list: gtk::ListBox = builder.get_object(NOTES_LIST_BOX_ID).unwrap();
  </code></pre>
  
  <p>And the event handler where I can't seem to add to <code>notes_list</code> (note that I've tried without the <code>clone!</code> macro, with strong and weak references, wrapping in <code>Rc</code> pointer, etc. but nothing seems to change):</p>
  
  <pre><code>open_menu_item.connect_activate(clone!(@strong state, @strong notes_list =&gt; move |_| {
      println!("Open database menu item activated");
  
      // Seemingly can't add to notes_list from within this closure???
      notes_list.add(&amp;gtk::Label::new(Some("TEST"))); // Doesn't work???
  
      let dialog = gtk::FileChooserDialog::with_buttons::&lt;gtk::Window&gt;(
          Some("Open database file"),
          None,
          gtk::FileChooserAction::Open,
          &amp;[("_Cancel", gtk::ResponseType::Cancel),
            ("_Open", gtk::ResponseType::Accept)]
      );
  
      dialog.connect_response(clone!(@weak state, @weak notes_list =&gt; move |this, res| {
          if res == gtk::ResponseType::Accept {
              let file = this.get_file().unwrap();
              let path_buf = file.get_path().unwrap();
  
              println!("Opening database file: {}", path_buf.as_path().display());
  
              let mut state = state.borrow_mut();
  
              state.db = database::database_in_file(path_buf.as_path()).ok();
              state.update_notes_list(&amp;notes_list);
          }
  
          this.close();
      }));
  
      dialog.show_all();
  }));
  </code></pre>
  
  <p>No error message is presented - the expected behaviour (i.e. the addition of a <code>gtk::Label</code> to the list box) does not occur.</p>
  
  <p>The full code of this module (and the rest of my messy code base): <a href="https://github.com/WiredSound/nos/blob/master/src/gui.rs" rel="nofollow noreferrer">https://github.com/WiredSound/nos/blob/master/src/gui.rs</a></p>
  
  <p>If anyone could help me figure this out then I'd really appreciate it, thanks.</p>