- 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
class Window : public mate::TrackableObject<Window>,
               public NativeWindowObserver {
 public:
  static mate::WrappableBase* New(v8::Isolate* isolate, mate::Arguments* args);
  static void BuildPrototype(v8::Isolate* isolate,
                             v8::Local<v8::ObjectTemplate> prototype);
  
  static v8::Local<v8::Value> From(v8::Isolate* isolate,
                                   NativeWindow* native_window);
  NativeWindow* window() const { return window_.get(); }
 protected:
  Window(v8::Isolate* isolate, const mate::Dictionary& options);
  ~Window() override;
  
  void WillCloseWindow(bool* prevent_default) override;
  void OnWindowClosed() override;
  void OnWindowBlur() override;
  void OnWindowFocus() override;
  void OnWindowShow() override;
  void OnWindowHide() override;
  void OnWindowMaximize() override;
  void OnWindowUnmaximize() override;
  void OnWindowMinimize() override;
  void OnWindowRestore() override;
  void OnWindowResize() override;
  void OnWindowMove() override;
  void OnWindowMoved() override;
  void OnWindowScrollTouchBegin() override;
  void OnWindowScrollTouchEnd() override;