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/api/atom_api_window.h
void SetTitle(const std::string& title);
  std::string GetTitle();
  void FlashFrame(bool flash);
  void SetSkipTaskbar(bool skip);
  void SetKiosk(bool kiosk);
  bool IsKiosk();
  void SetBackgroundColor(const std::string& color_name);
  void SetHasShadow(bool has_shadow);
  bool HasShadow();
  void FocusOnWebView();
  void BlurWebView();
  bool IsWebViewFocused();
  void SetRepresentedFilename(const std::string& filename);
  std::string GetRepresentedFilename();
  void SetDocumentEdited(bool edited);
  bool IsDocumentEdited();
  void SetIgnoreMouseEvents(bool ignore);
  void SetContentProtection(bool enable);
  void SetFocusable(bool focusable);
  void SetProgressBar(double progress, mate::Arguments* args);
  void SetOverlayIcon(const gfx::Image& overlay,
                      const std::string& description);
  bool SetThumbarButtons(mate::Arguments* args);
  void SetMenu(v8::Isolate* isolate, v8::Local<v8::Value> menu);
  void SetAutoHideMenuBar(bool auto_hide);
  bool IsMenuBarAutoHide();
  void SetMenuBarVisibility(bool visible);
  bool IsMenuBarVisible();
  void SetAspectRatio(double aspect_ratio, mate::Arguments* args);
  void PreviewFile(const std::string& path, mate::Arguments* args);