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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.h
public:
    virtual ~Observer() {}
    
    virtual void MenuWillClose() {}
  };
  explicit AtomMenuModel(Delegate* delegate);
  virtual ~AtomMenuModel();
  void AddObserver(Observer* obs) { observers_.AddObserver(obs); }
  void RemoveObserver(Observer* obs) { observers_.RemoveObserver(obs); }
  void SetRole(int index, const base::string16& role);
  base::string16 GetRoleAt(int index);
  bool GetAcceleratorAtWithParams(int index,
                                  bool use_default_accelerator,
                                  ui::Accelerator* accelerator) const;
  
  void MenuWillClose() override;
  using SimpleMenuModel::GetSubmenuModelAt;
  AtomMenuModel* GetSubmenuModelAt(int index);
 private:
  Delegate* delegate_;  
  std::map<int, base::string16> roles_;  
  base::ObserverList<Observer> observers_;
  DISALLOW_COPY_AND_ASSIGN(AtomMenuModel);
};
}  
#endif