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 SetVisibleOnAllWorkspaces(bool visible) = 0;
  virtual bool IsVisibleOnAllWorkspaces() = 0;
  
  virtual void SetVibrancy(const std::string& type);
  
  virtual void FocusOnWebView();
  virtual void BlurWebView();
  virtual bool IsWebViewFocused();
  
  virtual void SetAutoHideMenuBar(bool auto_hide);
  virtual bool IsMenuBarAutoHide();
  virtual void SetMenuBarVisibility(bool visible);
  virtual bool IsMenuBarVisible();
  
  double GetAspectRatio();
  gfx::Size GetAspectRatioExtraSize();
  virtual void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size);
  virtual void PreviewFile(const std::string& path,
                           const std::string& display_name);
  base::WeakPtr<NativeWindow> GetWeakPtr() {
    return weak_factory_.GetWeakPtr();
  }
  
  virtual void RequestToClosePage();
  
  virtual void CloseContents(content::WebContents* source);
  virtual void RendererUnresponsive(content::WebContents* source);
  virtual void RendererResponsive(content::WebContents* source);
  virtual void HandleKeyboardEvent(