Stop using deprecated Gtk::HBox

GtkHBox has been deprecated since GTK 3.2.  No further changes are
needed, since the default value of the GtkOrientable:orientation
property is GTK_ORIENTATION_HORIZONTAL.

src/main_window.hh

@@ -42,7 +42,7 @@ namespace Astroid {
       static int icon_size;
 
     private:
-      Gtk::HBox icons;
+      Gtk::Box icons;
       Gtk::Spinner poll_spinner;
       bool spinner_on = false;