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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_(new AtomMenuModel(this)),
      parent_(nullptr) {
  InitWith(isolate, wrapper);
}
Menu::~Menu() {
}
void Menu::AfterInit(v8::Isolate* isolate) {
  mate::Dictionary wrappable(isolate, GetWrapper());
  mate::Dictionary delegate;
  if (!wrappable.Get("delegate", &delegate))
    return;
  delegate.Get("isCommandIdChecked", &is_checked_);
  delegate.Get("isCommandIdEnabled", &is_enabled_);
  delegate.Get("isCommandIdVisible", &is_visible_);
  delegate.Get("getAcceleratorForCommandId", &get_accelerator_);
  delegate.Get("executeCommand", &execute_command_);
  delegate.Get("menuWillShow", &menu_will_show_);
}
bool Menu::IsCommandIdChecked(int command_id) const {
  return is_checked_.Run(command_id);
}
bool Menu::IsCommandIdEnabled(int command_id) const {