- 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
gfx::Rect GetBounds();
  void SetSize(int width, int height, mate::Arguments* args);
  std::vector<int> GetSize();
  void SetContentSize(int width, int height, mate::Arguments* args);
  std::vector<int> GetContentSize();
  void SetMinimumSize(int width, int height);
  std::vector<int> GetMinimumSize();
  void SetMaximumSize(int width, int height);
  std::vector<int> GetMaximumSize();
  void SetSheetOffset(double offset);
  void SetResizable(bool resizable);
  bool IsResizable();
  void SetMovable(bool movable);
  bool IsMovable();
  void SetMinimizable(bool minimizable);
  bool IsMinimizable();
  void SetMaximizable(bool maximizable);
  bool IsMaximizable();
  void SetFullScreenable(bool fullscreenable);
  bool IsFullScreenable();
  void SetClosable(bool closable);
  bool IsClosable();
  void SetAlwaysOnTop(bool top);
  bool IsAlwaysOnTop();
  void Center();
  void SetPosition(int x, int y, mate::Arguments* args);
  std::vector<int> GetPosition();
  void SetTitle(const std::string& title);
  std::string GetTitle();
  void FlashFrame(bool flash);