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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/ui/atom_menu_model.cc
int command_id = GetCommandIdAt(index);
  if (base::ContainsKey(roles_, command_id))
    return roles_[command_id];
  else
    return base::string16();
}
bool AtomMenuModel::GetAcceleratorAtWithParams(
    int index,
    bool use_default_accelerator,
    ui::Accelerator* accelerator) const {
  if (delegate_) {
    return delegate_->GetAcceleratorForCommandIdWithParams(
        GetCommandIdAt(index), use_default_accelerator, accelerator);
  }
  return false;
}
void AtomMenuModel::MenuWillClose() {
  ui::SimpleMenuModel::MenuWillClose();
  for (Observer& observer : observers_)
    observer.MenuWillClose();
}
AtomMenuModel* AtomMenuModel::GetSubmenuModelAt(int index) {
  return static_cast<AtomMenuModel*>(
      ui::SimpleMenuModel::GetSubmenuModelAt(index));
}
}