fix(macos): crash on removing menu item

[pewsheen]

Before this patch, all NSMenuItem registered to the outer Submenu id, but it's actually added into the inner Submenu's NSMenu. When removing the menu item, lines here will call the outer Submenu's NSMenu to remove the menu item that is added in the inner Submenu's NSMenu hence exception is thrown:

muda/src/platform_impl/macos/mod.rs

Lines 634 to 645 in 6454ea2

	if remove_from_cache {
	if let Some(ns_menu_items) = child
	.borrow_mut()
	.ns_menu_items
	.remove(&self.ns_menu.as_ref().unwrap().0)
	{
	for item in ns_menu_items {
	let () =
	unsafe { msg_send![self.ns_menu.as_ref().unwrap().1, removeItem: item] };
	}
	}
	}

• test code
  https://gist.github.com/pewsheen/12b6d63166cd9d0218c0b6c59d260502