Use gtk_widget_queue_draw() to redraw buttons.

* gtk2/candwin/horizontal-gtk.c
  - (button_clicked, uim_cand_win_gtk_set_index):
    Use gtk_widget_queue_draw() for redraw buttons instead of
    gtk_widget_unmap() and gtk_widget_map()
    because gtk_widget_unmap() and gtk_widget_map() are
    only for use in widget implementations
    and we can avoid warnings in GTK+3.
* gtk2/immodule/uim-cand-win-horizontal-gtk.c
  - (button_clicked, uim_cand_win_horizontal_gtk_set_index):
    Ditto.