Hmm..., it's a bit difficult for me.
Gtk::LevelBar generic constarcutor implement as follows?

```ruby


static VALUE
rg_initialize(int argc, VALUE *argv, VALUE self)
{
    VALUE min_value, max_value;
    GtkWidget* widget = NULL;

    rb_scan_args(argc, argv, "02", &min_value, &max_value);
    if (NIL_P(min_value) || NIL_P(max_value)) {
        widget = gtk_level_bar_new();
    } else {
        widget = gtk_level_bar_new_for_interval(NUM2DBL(min_value),
                                                NUM2DBL(max_value));
    }

    RBGTK_INITIALIZE(self, widget);
    return Qnil;
}
```

But this implement not work for `Gtk::LevelBar.new(min_value, max_value)`.