- 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
void OnWindowSwipe(const std::string& direction) override;
  void OnWindowEnterFullScreen() override;
  void OnWindowLeaveFullScreen() override;
  void OnWindowEnterHtmlFullScreen() override;
  void OnWindowLeaveHtmlFullScreen() override;
  void OnRendererUnresponsive() override;
  void OnRendererResponsive() override;
  void OnExecuteWindowsCommand(const std::string& command_name) override;
  #if defined(OS_WIN)
  void OnWindowMessage(UINT message, WPARAM w_param, LPARAM l_param) override;
  #endif
 private:
  
  void Close();
  void Focus();
  void Blur();
  bool IsFocused();
  void Show();
  void ShowInactive();
  void Hide();
  bool IsVisible();
  void Maximize();
  void Unmaximize();
  bool IsMaximized();
  void Minimize();
  void Restore();
  bool IsMinimized();
  void SetFullScreen(bool fullscreen);
  bool IsFullscreen();
  void SetBounds(const gfx::Rect& bounds, mate::Arguments* args);