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_mac.h
void SetBounds(const gfx::Rect& bounds, bool animate = false) override;
  gfx::Rect GetBounds() override;
  void SetContentSizeConstraints(
      const extensions::SizeConstraints& size_constraints) override;
  void SetResizable(bool resizable) override;
  bool IsResizable() override;
  void SetMovable(bool movable) override;
  void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size)
    override;
  void PreviewFile(const std::string& path, const std::string& display_name)
    override;
  bool IsMovable() override;
  void SetMinimizable(bool minimizable) override;
  bool IsMinimizable() override;
  void SetMaximizable(bool maximizable) override;
  bool IsMaximizable() override;
  void SetFullScreenable(bool fullscreenable) override;
  bool IsFullScreenable() override;
  void SetClosable(bool closable) override;
  bool IsClosable() override;
  void SetAlwaysOnTop(bool top, const std::string& level) override;
  bool IsAlwaysOnTop() override;
  void Center() override;
  void SetTitle(const std::string& title) override;
  std::string GetTitle() override;
  void FlashFrame(bool flash) override;
  void SetSkipTaskbar(bool skip) override;
  void SetKiosk(bool kiosk) override;
  bool IsKiosk() override;
  void SetBackgroundColor(const std::string& color_name) override;