- Electron version: All
- Operating system: OS X

Thanks @bengotow  for the y-offset, would it be possible to do a x-offset also? It may seem like an odd request but if you have a full-height sidebar menu then the sheet should be centered on the content pane. It's best understood with a screenshot (the dialog should not overlap the sidebar)...

<img width="902" alt="screen shot 2016-05-08 at 12 33 58-2" src="https://cloud.githubusercontent.com/assets/26971/15097682/39fb09ea-151b-11e6-812a-815e800b0cdf.png">

Probably would have been easier if I identified this potential issue in #4679 (sorry!).

Perhaps the best backwards compatible way to introduce this functionality is to keep the existing API but also accept an object with `x` and `y` values:

``` js
var toolbarRect = document.getElementById('toolbar').getBoundingClientRect();
var sidebarRect = document.getElementById('sidebar').getBoundingClientRect();

win.setSheetOffset({
  x: sidebarRect.width / 2,
  y: toolbarRect.height
});
```

I'm happy to have a go implementing this myself but I haven't hacked on the codebase before and since @bengotow implemented the original feature then perhaps he is better placed to add this functionality?

atom/browser/native_window.h
virtual bool IsVisibleOnAllWorkspaces() = 0;
  
  virtual void FocusOnWebView();
  virtual void BlurWebView();
  virtual bool IsWebViewFocused();
  
  
  virtual void CapturePage(const gfx::Rect& rect,
                           const CapturePageCallback& callback);
  
  virtual void ShowDefinitionForSelection();
  
  virtual void SetAutoHideMenuBar(bool auto_hide);
  virtual bool IsMenuBarAutoHide();
  virtual void SetMenuBarVisibility(bool visible);
  virtual bool IsMenuBarVisible();
  
  double GetAspectRatio();
  gfx::Size GetAspectRatioExtraSize();
  void SetAspectRatio(double aspect_ratio, const gfx::Size& extra_size);
  base::WeakPtr<NativeWindow> GetWeakPtr() {
    return weak_factory_.GetWeakPtr();
  }
  
  virtual void RequestToClosePage();
  
  virtual void CloseContents(content::WebContents* source);
  virtual void RendererUnresponsive(content::WebContents* source);
  virtual void RendererResponsive(content::WebContents* source);
  virtual void HandleKeyboardEvent(