- Electron version:1.3.3
- Operating system:win10/mac

the Tray instance events shold provide the tray hover event. 
`tray.setToolTip` has limitations when do more operation.

atom/browser/ui/tray_icon.h
ALWAYS,  
    NEVER,  
    SELECTION  
  };
  virtual void SetHighlightMode(HighlightMode mode);
  
  
  virtual void DisplayBalloon(ImageType icon,
                              const base::string16& title,
                              const base::string16& contents);
  
  virtual void PopUpContextMenu(const gfx::Point& pos,
                                AtomMenuModel* menu_model);
  
  virtual void SetContextMenu(AtomMenuModel* menu_model) = 0;
  
  virtual gfx::Rect GetBounds();
  void AddObserver(TrayIconObserver* obs) { observers_.AddObserver(obs); }