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 SetHasShadow(bool has_shadow) override;
  bool HasShadow() override;
  void SetRepresentedFilename(const std::string& filename) override;
  std::string GetRepresentedFilename() override;
  void SetDocumentEdited(bool edited) override;
  bool IsDocumentEdited() override;
  void SetIgnoreMouseEvents(bool ignore) override;
  void SetContentProtection(bool enable) override;
  void SetParentWindow(NativeWindow* parent) override;
  gfx::NativeWindow GetNativeWindow() override;
  gfx::AcceleratedWidget GetAcceleratedWidget() override;
  void SetProgressBar(double progress, const ProgressState state) override;
  void SetOverlayIcon(const gfx::Image& overlay,
                      const std::string& description) override;
  void SetVisibleOnAllWorkspaces(bool visible) override;
  bool IsVisibleOnAllWorkspaces() override;
  void SetVibrancy(const std::string& type) override;
  
  void OnInputEvent(const blink::WebInputEvent& event) override;
  
  void RenderViewHostChanged(content::RenderViewHost* old_host,
                             content::RenderViewHost* new_host) override;
  
  void UpdateDraggableRegionViews() {
    UpdateDraggableRegionViews(draggable_regions_);
  }
  
  void SetStyleMask(bool on, NSUInteger flag);
  void SetCollectionBehavior(bool on, NSUInteger flag);
  enum TitleBarStyle {