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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/tray_icon_cocoa.h
#ifndef ATOM_BROWSER_UI_TRAY_ICON_COCOA_H_
#define ATOM_BROWSER_UI_TRAY_ICON_COCOA_H_
#import <Cocoa/Cocoa.h>
#include <string>
#include "atom/browser/ui/tray_icon.h"
#include "base/mac/scoped_nsobject.h"
@class AtomMenuController;
@class StatusItemView;