<!--
Thanks for opening an issue! A few things to keep in mind:

- The issue tracker is only for bugs and feature requests.
- Before reporting a bug, please try reproducing your issue against
  the latest version of Electron.
- If you need general advice, join our Slack: http://atom-slack.herokuapp.com
-->

* Electron version: 1.6.2
* Operating system: macOS

### Expected behavior
Allow users to register a callback for menu open and close events.
```javascript
const menu = new Menu()

menu.onOpen(() => { /* Do something when the menu opens */ })
menu.onClose(() => { /* Do something when the menu closes */ })
```
<!-- What do you think should happen? -->

### Actual behavior

Not supported


NSMenuDelegate exposes these events: https://developer.apple.com/reference/appkit/nsmenudelegate

Not sure about Windows or Linux

atom/browser/api/atom_api_menu.h
base::Callback<bool(int)> is_checked_;
  base::Callback<bool(int)> is_enabled_;
  base::Callback<bool(int)> is_visible_;
  base::Callback<v8::Local<v8::Value>(int, bool)> get_accelerator_;
  base::Callback<void(v8::Local<v8::Value>, int)> execute_command_;
  base::Callback<void()> menu_will_show_;
  DISALLOW_COPY_AND_ASSIGN(Menu);
};
}  
}  
namespace mate {
template<>
struct Converter<atom::AtomMenuModel*> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,
                     atom::AtomMenuModel** out) {
    
    if (val->IsNull()) {
      *out = nullptr;
      return true;
    }
    atom::api::Menu* menu;
    if (!Converter<atom::api::Menu*>::FromV8(isolate, val, &menu))
      return false;
    *out = menu->model();
    return true;
  }
};
}  
#endif