Menu creation improvements

[probablykasper]

Currently, this is how you would define a fairly standard menu. It's pretty awkward because you can't completely use the builder pattern if you have any platform differences:

.menu({
  let mut menu = Menu::new();
  #[cfg(target_os = "macos")]
  {
    menu = menu.add_submenu(Submenu::new(
      &ctx.package_info().name,
      Menu::new()
        .add_native_item(MenuItem::About(ctx.package_info().name.clone()))
        .add_native_item(MenuItem::Separator)
        .add_item(CustomMenuItem::new("Preferences...", "Preferences...").accelerator("CmdOrCtrl+,"))
        .add_native_item(MenuItem::Separator),
        // ...
    ))
  }
  menu = menu
    .add_submenu(/* File submenu */)
    .add_submenu(/* Edit submenu */)
    .add_submenu(/* View submenu */)
    .add_submenu(/* Window submenu */)
    .add_submenu(/* Help submenu */);
  menu
})

If you could pass a list of submenus instead, it becomes more intuitive because you can directly add attributes to the items:

.menu(Menu::new(vec![
  #[cfg(target_os = "macos")]
  MenuItem::Submenu(Submenu::new(
    &ctx.package_info().name,
    Menu::new(vec![
      MenuItem::About("My App".to_string()),
      MenuItem::Separator,
      MenuItem::Custom(CustomMenuItem::new("Preferences...", "Preferences...").accelerator("CmdOrCtrl+,")),
      MenuItem::Separator,
    ]),
  )),
  MenuItem::Submenu(Submenu::new(/* File submenu */)),
  MenuItem::Submenu(Submenu::new(/* Edit submenu */)),
  MenuItem::Submenu(Submenu::new(/* View submenu */)),
  MenuItem::Submenu(Submenu::new(/* Window submenu */)),
  MenuItem::Submenu(Submenu::new(/* Help submenu */)),
]))

The differences here are:

• Menu::new() takes Vec<MenuItem>
• New MenuItem variant: MenuItem::Custom(CustomMenuItem)
• New MenuItem variant: MenuItem::Submenu(Submenu)

[probablykasper]

I'll attempt a PR if this seems good

[amrbashir]

I definitely like this approach but I think this should be done in Tao ? I was planning on redoing our menu system tbh and this feels like a good improvement, don't you think @lemarier ?

[probablykasper]

I'm really liking this one:

.menu(Menu::new([
  #[cfg(target_os = "macos")]
  MenuItem::new_submenu(
    &ctx.package_info().name,
    [
      MenuItem::About("My App".into()),
      MenuItem::Separator,
      CustomMenuItem::new("Preferences...", "Preferences...").accelerator("CmdOrCtrl+,").into(),
      MenuItem::Separator,
    ],
  )),
  MenuItem::new_submenu([/* File submenu items */]),
  MenuItem::new_submenu([/* Edit submenu items */]),
  MenuItem::new_submenu([/* View submenu items */]),
  MenuItem::new_submenu([/* Window submenu items */]),
  MenuItem::new_submenu([/* Help submenu items */]),
]))

[probablykasper]

Would it be a bad idea to delete Menu and use Vec<MenuItem> directly instead?