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* 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.cc
v8::Local<v8::Context> context, void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  Menu::SetConstructor(isolate, base::Bind(&Menu::New));
  mate::Dictionary dict(isolate, exports);
  dict.Set("Menu", Menu::GetConstructor(isolate)->GetFunction());
#if defined(OS_MACOSX)
  dict.SetMethod("setApplicationMenu", &Menu::SetApplicationMenu);
  dict.SetMethod("sendActionToFirstResponder",
                 &Menu::SendActionToFirstResponder);
#endif
}
}  
NODE_MODULE_CONTEXT_AWARE_BUILTIN(atom_browser_menu, Initialize)