- 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
SetSize(ContentSizeToWindowSize(size), animate);
}
gfx::Size NativeWindow::GetContentSize() {
  return WindowSizeToContentSize(GetSize());
}
void NativeWindow::SetSizeConstraints(
    const extensions::SizeConstraints& window_constraints) {
  extensions::SizeConstraints content_constraints;
  if (window_constraints.HasMaximumSize())
    content_constraints.set_maximum_size(
        WindowSizeToContentSize(window_constraints.GetMaximumSize()));
  if (window_constraints.HasMinimumSize())
    content_constraints.set_minimum_size(
        WindowSizeToContentSize(window_constraints.GetMinimumSize()));
  SetContentSizeConstraints(content_constraints);
}
extensions::SizeConstraints NativeWindow::GetSizeConstraints() {
  extensions::SizeConstraints content_constraints = GetContentSizeConstraints();
  extensions::SizeConstraints window_constraints;
  if (content_constraints.HasMaximumSize())
    window_constraints.set_maximum_size(
        ContentSizeToWindowSize(content_constraints.GetMaximumSize()));
  if (content_constraints.HasMinimumSize())
    window_constraints.set_minimum_size(
        ContentSizeToWindowSize(content_constraints.GetMinimumSize()));
  return window_constraints;
}
void NativeWindow::SetContentSizeConstraints(