In 1.4.5, Electron added a new API (`previewFile`) that allows you to open a quicklook preview panel. However, there isn't an API for closing the preview panel that is opened.

QLPreviewPanel is a subclass of NSWindow - using Mac OS X APIs, it's pretty easy to close the shared Quicklook preview panel.

We need this functionality because the panel does not close automatically when the window is blurred, or when another window is opened in the application (https://github.com/nylas/N1/issues/3021). Other user action, such as moving focus to a message that doesn't have quicklook attachments should also probably close the panel.

I'm not sure what a good name for this API would be. Maybe `dismissFilePreview` or `unpreviewFile`? 
atom/browser/native_window.h
virtual void FlashFrame(bool flash) = 0;
  virtual void SetSkipTaskbar(bool skip) = 0;
  virtual void SetKiosk(bool kiosk) = 0;
  virtual bool IsKiosk() = 0;
  virtual void SetBackgroundColor(const std::string& color_name) = 0;
  virtual void SetHasShadow(bool has_shadow) = 0;
  virtual bool HasShadow() = 0;
  virtual void SetRepresentedFilename(const std::string& filename);
  virtual std::string GetRepresentedFilename();
  virtual void SetDocumentEdited(bool edited);
  virtual bool IsDocumentEdited();
  virtual void SetIgnoreMouseEvents(bool ignore) = 0;
  virtual void SetContentProtection(bool enable) = 0;
  virtual void SetFocusable(bool focusable);
  virtual void SetMenu(AtomMenuModel* menu);
  virtual void SetParentWindow(NativeWindow* parent);
  virtual gfx::NativeWindow GetNativeWindow() = 0;
  virtual gfx::AcceleratedWidget GetAcceleratedWidget() = 0;
  
  enum ProgressState {
    PROGRESS_NONE,               
    PROGRESS_INDETERMINATE,      
    PROGRESS_ERROR,              
    PROGRESS_PAUSED,             
    PROGRESS_NORMAL,             
  };
  virtual void SetProgressBar(double progress,
                              const ProgressState state) = 0;
  virtual void SetOverlayIcon(const gfx::Image& overlay,
                              const std::string& description) = 0;