Use GtkGrid instead of deprecated GtkTable.

* gtk2/candwin/horizontal-gtk.c
  - (candidate_window_init, assign_cellbutton, update_table_button,
     uim_cand_win_gtk_set_page, show_table):
    Use GtkGrid instead of GtkTable.
* gtk2/immodule/uim-cand-win-horizontal-gtk.c
  - (uim_cand_win_horizontal_gtk_init, assign_cellbutton
     update_table_button, uim_cand_win_horizontal_gtk_set_page,
     show_table):
    Ditto.