patch 8.0.1558: no right-click menu in a terminal

Problem:    No right-click menu in a terminal.
Solution:   Implement the right click menu for the terminal.
diff --git a/src/proto/menu.pro b/src/proto/menu.pro
index bd3d26b..4c8e710 100644
--- a/src/proto/menu.pro
+++ b/src/proto/menu.pro
@@ -12,12 +12,13 @@
 int menu_is_child_of_popup(vimmenu_T *menu);
 int menu_is_toolbar(char_u *name);
 int menu_is_separator(char_u *name);
+void show_popupmenu(void);
 int check_menu_pointer(vimmenu_T *root, vimmenu_T *menu_to_check);
 void gui_create_initial_menus(vimmenu_T *menu);
 void gui_update_menus(int modes);
 int gui_is_menu_shortcut(int key);
-void gui_show_popupmenu(void);
 void gui_mch_toggle_tearoffs(int enable);
+void execute_menu(exarg_T *eap, vimmenu_T *menu);
 void ex_emenu(exarg_T *eap);
 void winbar_click(win_T *wp, int col);
 vimmenu_T *gui_find_menu(char_u *path_name);