Remove platform-independent menu definitions.

Remove platform-independent menu definitions.
(menu_items menu_items_inuse, menu_items_allocated)
(menu_items_used, menu_items_n_panes)
(menu_items_submenu_depth): Move to keyboard.h.
(init_menu_items, finish_menu_items, unuse_menu_items)
(discard_menu_items, restore_menu_items, save_menu_items)
(grow_menu_items, push_submenu_start, push_submenu_end)
(push_left_right_boundary, push_menu_pane, push_menu_item)
(keymap_panes, single_keymap_panes, single_menu_item)
(list_of_panes, list_of_items, find_and_call_menu_selection)
(xmalloc_widget_value, free_menubar_widget_value_tree)
(parse_single_submenu, digest_single_submenu)
(update_submenu_strings): Move to menu.c.