- 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.h
bool IsWindowMessageHooked(UINT message);
  void UnhookWindowMessage(UINT message);
  void UnhookAllWindowMessages();
#endif
#if defined(OS_MACOSX)
  void ShowDefinitionForSelection();
#endif
#if defined(TOOLKIT_VIEWS)
  void SetIcon(mate::Handle<NativeImage> icon);
#endif
  void SetVisibleOnAllWorkspaces(bool visible);
  bool IsVisibleOnAllWorkspaces();
  int32_t ID() const;
  v8::Local<v8::Value> WebContents(v8::Isolate* isolate);
#if defined(OS_WIN)
  typedef std::map<UINT, MessageCallback> MessageCallbackMap;
  MessageCallbackMap messages_callback_map_;
#endif
  v8::Global<v8::Value> web_contents_;
  v8::Global<v8::Value> menu_;
  api::WebContents* api_web_contents_;
  scoped_ptr<NativeWindow> window_;
  DISALLOW_COPY_AND_ASSIGN(Window);
};
}  
}  
namespace mate {
template<>
struct Converter<atom::NativeWindow*> {
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,