file chooser: Improve keynav

Some internal containers were erroneously taking focus, interrupting
the flow of tab keynav, and using arrow keynav to go from the file
list to the sidebar did not work anymore, after the recent sidebar
rewrite.

gtk/gtkfilechooserwidget.c

@@ -1227,8 +1227,8 @@ browse_files_key_press_event_cb (GtkWidget   *widget,
 
   if (key_is_left_or_right (event))
     {
-      gtk_widget_grab_focus (priv->places_sidebar);
-      return TRUE;
+      if (gtk_widget_child_focus (priv->places_sidebar, GTK_DIR_LEFT))
+        return TRUE;
     }
 
   if ((event->keyval == GDK_KEY_Return