Use GtkGrid instead of deprecated GtkTable.

* gtk2/dict/word-win-gtk.c
  - (word_window_necessary_create, word_window_additional_create):
    Use GtkGrid instead of GtkTable.
* gtk2/pad/ja.c
  - (buttontable_create): Ditto.
* gtk2/pref/gtk-custom-widgets.c
  - (choose_olist_clicked_cb): Ditto.