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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
namespace atom {
class TrayIconCocoa : public TrayIcon,
                      public AtomMenuModel::Observer {
 public:
  TrayIconCocoa();
  virtual ~TrayIconCocoa();
  void SetImage(const gfx::Image& image) override;
  void SetPressedImage(const gfx::Image& image) override;
  void SetToolTip(const std::string& tool_tip) override;
  void SetTitle(const std::string& title) override;
  void SetHighlightMode(TrayIcon::HighlightMode mode) override;
  void PopUpContextMenu(const gfx::Point& pos,
                        AtomMenuModel* menu_model) override;
  void SetContextMenu(AtomMenuModel* menu_model) override;
  gfx::Rect GetBounds() override;
 protected:
  
  void MenuWillClose() override;
 private:
  
  base::scoped_nsobject<StatusItemView> status_item_view_;
  
  base::scoped_nsobject<AtomMenuController> menu_;
  
  AtomMenuModel* menu_model_;  
  DISALLOW_COPY_AND_ASSIGN(TrayIconCocoa);
};
}  
#endif