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 Focus(bool focus) = 0;
  virtual bool IsFocused() = 0;
  virtual void Show() = 0;
  virtual void ShowInactive() = 0;
  virtual void Hide() = 0;
  virtual bool IsVisible() = 0;
  virtual bool IsEnabled() = 0;
  virtual void Maximize() = 0;
  virtual void Unmaximize() = 0;
  virtual bool IsMaximized() = 0;
  virtual void Minimize() = 0;
  virtual void Restore() = 0;
  virtual bool IsMinimized() = 0;
  virtual void SetFullScreen(bool fullscreen) = 0;
  virtual bool IsFullscreen() const = 0;
  virtual void SetBounds(const gfx::Rect& bounds, bool animate = false) = 0;
  virtual gfx::Rect GetBounds() = 0;
  virtual void SetSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetSize();
  virtual void SetPosition(const gfx::Point& position, bool animate = false);
  virtual gfx::Point GetPosition();
  virtual void SetContentSize(const gfx::Size& size, bool animate = false);
  virtual gfx::Size GetContentSize();
  virtual void SetContentBounds(const gfx::Rect& bounds, bool animate = false);
  virtual gfx::Rect GetContentBounds();
  virtual void SetSizeConstraints(