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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.h
public AtomMenuModel::Delegate {
 public:
  static mate::WrappableBase* New(mate::Arguments* args);
  static void BuildPrototype(v8::Isolate* isolate,
                             v8::Local<v8::FunctionTemplate> prototype);
#if defined(OS_MACOSX)
  
  static void SetApplicationMenu(Menu* menu);
  
  static void SendActionToFirstResponder(const std::string& action);
#endif
  AtomMenuModel* model() const { return model_.get(); }
 protected:
  Menu(v8::Isolate* isolate, v8::Local<v8::Object> wrapper);
  ~Menu() override;
  
  void AfterInit(v8::Isolate* isolate) override;
  
  bool IsCommandIdChecked(int command_id) const override;
  bool IsCommandIdEnabled(int command_id) const override;
  bool IsCommandIdVisible(int command_id) const override;
  bool GetAcceleratorForCommandIdWithParams(
      int command_id,
      bool use_default_accelerator,
      ui::Accelerator* accelerator) const override;
  void ExecuteCommand(int command_id, int event_flags) override;
  void MenuWillShow(ui::SimpleMenuModel* source) override;
  virtual void PopupAt(Window* window, int x, int y, int positioning_item,
                       const base::Closure& callback) = 0;