`gtk_level_var_new_for_interval()` is constructor. So we should bind it as instance method.
It should be able to use as `Gtk::LevelBar.new(min_value, max_value)` instead of `Gtk::LevelBar.new.for_interval(min_value, max_value)`.

`Gtk::LevelBar.new` should work with both `Gtk::LevelBar.new()` and `Gtk::LevelBar.new(min_value, max_value)`.

See `rbgtk-label.c` about how to create a generic constructor.