
- slow thread change
    It's is performed on Gtk+ timeout method to change ruby threads. Its
    interval time, 100ms, is too long to change threads.
    But if you set it to 1ms, then Gtk+ event handler is too late...

- Gtk::OptionMenu#children does't return correct object.
    If you call Gtk::OptionMenu#children before calling Gtk::OptionMenu#child,
    correspoding object to Gtk::OptionMenu's child is not created yet,
    and so Gtk::OptionMenu#children does't return child widget.

- memory management for GDK object is imperfect.
