(Fx_popup_menu): If POSITION is nil,

(Fx_popup_menu): If POSITION is nil,
don't require an open X connection.
(single_keymap_panes, keymap_panes, menu_item_enabled_p): New arg NOTREAL.
(Fx_popup_menu): Pass new arg (1 if POSITION is nil).
(menu_item_enabled_p): If NOTREAL, always return t.

(single_keymap_panes) [!USE_X_TOOLKIT]:
Append > to item_string if submenu.

[USE_X_TOOLKIT]: Display submenus in Xt style.
(menu_items): Record where submenus start and end.
(menu_items_submenu_depth): New variable.
(init_menu_items): Init it.
(push_submenu_start, push_submenu_end): New functions.
(grow_menu_items): New function.
(push_menu_pane, push_menu_item): Use it.
(push_menu_pane): Increment menu_items_n_panes only if depth is 0.
(single_keymap_panes) [USE_X_TOOLKIT]: Record submenus in menu_items.
(xmenu_show) [USE_X_TOOLKIT]: Give submenus to toolkit.