Add separator / grayed flag support

[ZX-80]

Problem: Windows menus had not yet implemented separators or greyed out menus when disabled.

Solution:

• Set the MF_GRAYED flag if disabled
• Append a separator if the ID is MENU_ID_SEPARATOR

To toggle the state, an enable_menu function was added. You use it like this. Let me know if that fits the style of this project or if stubs are needed for other os modules

// Menu 1
let mut menu_1 = Menu::new("Menu 1").unwrap();
menu_1.add_item("Item 1", 1).build();
menu_1.add_item("Item 2", 2).build();

// Menu 2
let mut menu_2 = Menu::new("Menu 2").unwrap();
menu_2.add_item("Item 1", 3).build();
menu_2.add_item("Item 2", 4).build();

let menu_1_handle = window.add_menu(&menu_1 );
let menu_2_handle = window.add_menu(&menu_2 );

// Disable the second item in menu 1
window.enable_menu(Some(menu_1_handle ), 1, false);
// Disable menu 2
window.enable_menu(None, 1, false);

[emoon]

Thanks!