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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
#include "atom/browser/api/atom_api_menu.h"
#include "atom/browser/native_window.h"
#include "atom/common/native_mate_converters/accelerator_converter.h"
#include "atom/common/native_mate_converters/callback.h"
#include "atom/common/native_mate_converters/image_converter.h"
#include "atom/common/native_mate_converters/string16_converter.h"
#include "native_mate/constructor.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "atom/common/node_includes.h"
namespace atom {
namespace api {
Menu::Menu(v8::Isolate* isolate, v8::Local<v8::Object> wrapper)