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
const extensions::SizeConstraints& size_constraints);
  virtual extensions::SizeConstraints GetSizeConstraints();
  virtual void SetContentSizeConstraints(
      const extensions::SizeConstraints& size_constraints);
  virtual extensions::SizeConstraints GetContentSizeConstraints();
  virtual void SetMinimumSize(const gfx::Size& size);
  virtual gfx::Size GetMinimumSize();
  virtual void SetMaximumSize(const gfx::Size& size);
  virtual gfx::Size GetMaximumSize();
  virtual void SetSheetOffset(const double offsetX, const double offsetY);
  virtual double GetSheetOffsetX();
  virtual double GetSheetOffsetY();
  virtual void SetResizable(bool resizable) = 0;
  virtual bool IsResizable() = 0;
  virtual void SetMovable(bool movable) = 0;
  virtual bool IsMovable() = 0;
  virtual void SetMinimizable(bool minimizable) = 0;
  virtual bool IsMinimizable() = 0;
  virtual void SetMaximizable(bool maximizable) = 0;
  virtual bool IsMaximizable() = 0;
  virtual void SetFullScreenable(bool fullscreenable) = 0;
  virtual bool IsFullScreenable() = 0;
  virtual void SetClosable(bool closable) = 0;
  virtual bool IsClosable() = 0;
  virtual void SetAlwaysOnTop(bool top,
                              const std::string& level = "floating") = 0;
  virtual bool IsAlwaysOnTop() = 0;
  virtual void Center() = 0;
  virtual void SetTitle(const std::string& title) = 0;
  virtual std::string GetTitle() = 0;