More examples for menu handling

[eirnym]

I'd like to request 2 examples (could be a single app) for menu item handling

• Enable/disable menu item on demand as application state changes
• Handling menu modification on modifier key pressed. This is more complex example even in native ObjC, but I'd like to see, how it's better to make this happen using this library

Example for the second one is when I press Option key Save with modifier "Cmd-S" become Save As with modifier Opt-Cmd-S