- 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/native_window.cc
int x = -1, y = -1;
  bool center;
  if (options.Get(options::kX, &x) && options.Get(options::kY, &y)) {
    SetPosition(gfx::Point(x, y));
  } else if (options.Get(options::kCenter, &center) && center) {
    Center();
  }
  
  extensions::SizeConstraints size_constraints(GetContentSizeConstraints());
  int min_height = 0, min_width = 0;
  if (options.Get(options::kMinHeight, &min_height) |
      options.Get(options::kMinWidth, &min_width)) {
    size_constraints.set_minimum_size(gfx::Size(min_width, min_height));
  }
  int max_height = INT_MAX, max_width = INT_MAX;
  if (options.Get(options::kMaxHeight, &max_height) |
      options.Get(options::kMaxWidth, &max_width)) {
    size_constraints.set_maximum_size(gfx::Size(max_width, max_height));
  }
  bool use_content_size = false;
  options.Get(options::kUseContentSize, &use_content_size);
  if (use_content_size) {
    SetContentSizeConstraints(size_constraints);
  } else {
    SetSizeConstraints(size_constraints);
  }