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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
model_->SetRole(index, role);
}
void Menu::Clear() {
  model_->Clear();
}
int Menu::GetIndexOfCommandId(int command_id) {
  return model_->GetIndexOfCommandId(command_id);
}
int Menu::GetItemCount() const {
  return model_->GetItemCount();
}
int Menu::GetCommandIdAt(int index) const {
  return model_->GetCommandIdAt(index);
}
base::string16 Menu::GetLabelAt(int index) const {
  return model_->GetLabelAt(index);
}
base::string16 Menu::GetSublabelAt(int index) const {
  return model_->GetSublabelAt(index);
}
bool Menu::IsItemCheckedAt(int index) const {
  return model_->IsItemCheckedAt(index);
}
bool Menu::IsEnabledAt(int index) const {
  return model_->IsEnabledAt(index);
}
bool Menu::IsVisibleAt(int index) const {
  return model_->IsVisibleAt(index);
}
void Menu::BuildPrototype(v8::Isolate* isolate,