chore(example): add support for native icons in submenu items

Introduce a new method `set_native_icon` to allow changing the menu
item icon to a native image. This method currently supports macOS,
while remaining unsupported on Windows and Linux platforms.
The addition enhances the customization options for submenu items.

src/items/submenu.rs

@@ -4,7 +4,7 @@
 
 use std::{cell::RefCell, mem, rc::Rc};
 
-use crate::{dpi::Position, sealed::IsMenuItemBase, util::AddOp, ContextMenu, Icon, IsMenuItem, MenuId, MenuItemKind};
+use crate::{dpi::Position, sealed::IsMenuItemBase, util::AddOp, ContextMenu, Icon, IsMenuItem, MenuId, MenuItemKind, NativeIcon};
 
 /// A menu that can be added to a [`Menu`] or another [`Submenu`].
 ///
@@ -210,6 +210,16 @@ impl Submenu {
     pub fn set_icon(&self, icon: Option<Icon>) {
         self.inner.borrow_mut().set_icon(icon)
     }
+
+    /// Change this menu item icon to a native image or remove it.
+    ///
+    /// ## Platform-specific:
+    ///
+    /// - **Windows / Linux**: Unsupported.
+    pub fn set_native_icon(&self, _icon: Option<NativeIcon>) {
+        #[cfg(target_os = "macos")]
+        self.inner.borrow_mut().set_native_icon(_icon)
+    }
 }
 
 impl ContextMenu for Submenu {