- 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/api/atom_api_window.cc
mate::Dictionary web_preferences = mate::Dictionary::CreateEmpty(isolate);
  options.Get(options::kWebPreferences, &web_preferences);
  
  v8::Local<v8::Value> value;
  if (options.Get(options::kBackgroundColor, &value))
    web_preferences.Set(options::kBackgroundColor, value);
  
  auto web_contents = WebContents::Create(isolate, web_preferences);
  web_contents_.Reset(isolate, web_contents.ToV8());
  api_web_contents_ = web_contents.get();
  
  mate::Dictionary(isolate, web_contents->GetWrapper()).Set(
      "browserWindowOptions", options);
  
  window_.reset(NativeWindow::Create(web_contents->managed_web_contents(),
                                     options));
  web_contents->SetOwnerWindow(window_.get());
  window_->InitFromOptions(options);
  window_->AddObserver(this);
  AttachAsUserData(window_.get());
#if defined(TOOLKIT_VIEWS)
  
  mate::Handle<NativeImage> icon;
  if (options.Get(options::kIcon, &icon))
    SetIcon(icon);
#endif
}
Window::~Window() {