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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
#ifndef ATOM_BROWSER_UI_ATOM_MENU_MODEL_H_
#define ATOM_BROWSER_UI_ATOM_MENU_MODEL_H_
#include <map>
#include "base/observer_list.h"
#include "ui/base/models/simple_menu_model.h"
namespace atom {
class AtomMenuModel : public ui::SimpleMenuModel {
 public:
  class Delegate : public ui::SimpleMenuModel::Delegate {
   public:
    virtual ~Delegate() {}
    virtual bool GetAcceleratorForCommandIdWithParams(
        int command_id,
        bool use_default_accelerator,
        ui::Accelerator* accelerator) const = 0;
   private:
    
    bool GetAcceleratorForCommandId(int command_id,
                                    ui::Accelerator* accelerator) const {
      return GetAcceleratorForCommandIdWithParams(
          command_id, false, accelerator);
    }
  };
  class Observer {