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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
virtual void ClosePopupAt(int32_t window_id) = 0;
  std::unique_ptr<AtomMenuModel> model_;
  Menu* parent_;
 private:
  void InsertItemAt(int index, int command_id, const base::string16& label);
  void InsertSeparatorAt(int index);
  void InsertCheckItemAt(int index,
                         int command_id,
                         const base::string16& label);
  void InsertRadioItemAt(int index,
                         int command_id,
                         const base::string16& label,
                         int group_id);
  void InsertSubMenuAt(int index,
                       int command_id,
                       const base::string16& label,
                       Menu* menu);
  void SetIcon(int index, const gfx::Image& image);
  void SetSublabel(int index, const base::string16& sublabel);
  void SetRole(int index, const base::string16& role);
  void Clear();
  int GetIndexOfCommandId(int command_id);
  int GetItemCount() const;
  int GetCommandIdAt(int index) const;
  base::string16 GetLabelAt(int index) const;
  base::string16 GetSublabelAt(int index) const;
  bool IsItemCheckedAt(int index) const;
  bool IsEnabledAt(int index) const;
  bool IsVisibleAt(int index) const;