MF_POPUP : 用于插入一个子菜单
Specifies that the menu item opens a drop-down menu or submenu. The uIDNewItem parameter specifies a handle to the drop-down menu or submenu. This flag is used to add a menu name to a menu bar, or a menu item that opens a submenu to a drop-down menu, submenu, or shortcut menu.
MF_STRING : 用于添加一个菜单项
Specifies that the menu item is a text string; the lpNewItem parameter is a pointer to the string.