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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_->InsertSeparatorAt(index, ui::NORMAL_SEPARATOR);
}
void Menu::InsertCheckItemAt(int index,
                             int command_id,
                             const base::string16& label) {
  model_->InsertCheckItemAt(index, command_id, label);
}
void Menu::InsertRadioItemAt(int index,
                             int command_id,
                             const base::string16& label,
                             int group_id) {
  model_->InsertRadioItemAt(index, command_id, label, group_id);
}
void Menu::InsertSubMenuAt(int index,
                           int command_id,
                           const base::string16& label,
                           Menu* menu) {
  menu->parent_ = this;
  model_->InsertSubMenuAt(index, command_id, label, menu->model_.get());
}
void Menu::SetIcon(int index, const gfx::Image& image) {
  model_->SetIcon(index, image);
}
void Menu::SetSublabel(int index, const base::string16& sublabel) {
  model_->SetSublabel(index, sublabel);
}
void Menu::SetRole(int index, const base::string16& role) {