[feat] Enhance Tauri Tray Menus: Support Icons for Submenus and Checkable Items

[mediarl]

Describe the problem

The tray menu system has limitations in supporting icons for menu items. Currently:

• Menu items cannot have icons if they include submenus.
• Submenu items themselves cannot have icons.
• Checked menu items cannot display icons.

These limitations restrict the ability to create visually rich and user-friendly tray menus. On macOS, applications like Klack demonstrate that all these features are possible, enhancing the overall user experience.

Describe the solution you'd like

I’d like Tauri v2 to enhance its tray menu functionality by:

• Supporting icons for menu items that include submenus.
• Allowing submenu items to have icons.
• Enabling icons for checkable menu items.

This functionality would allow developers to build visually consistent and feature-complete tray menus, leveraging the design capabilities of macOS.

Alternatives considered

1. Custom tray menu implementations using native bindings or platform-specific workarounds, which significantly increase development complexity.

2. Accepting the current limitations and avoiding icons in menus, which compromises design and usability.
   Both alternatives are less than ideal compared to having first-class support directly in Tauri.

Additional context

No response