- 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.cc
window_->SetDocumentEdited(edited);
}
bool Window::IsDocumentEdited() {
  return window_->IsDocumentEdited();
}
void Window::SetIgnoreMouseEvents(bool ignore) {
  return window_->SetIgnoreMouseEvents(ignore);
}
void Window::CapturePage(mate::Arguments* args) {
  gfx::Rect rect;
  base::Callback<void(const gfx::Image&)> callback;
  if (!(args->Length() == 1 && args->GetNext(&callback)) &&
      !(args->Length() == 2 && args->GetNext(&rect)
                            && args->GetNext(&callback))) {
    args->ThrowError();
    return;
  }
  window_->CapturePage(
      rect, base::Bind(&OnCapturePageDone, args->isolate(), callback));
}
void Window::SetProgressBar(double progress) {
  window_->SetProgressBar(progress);
}
void Window::SetOverlayIcon(const gfx::Image& overlay,
                            const std::string& description) {
  window_->SetOverlayIcon(overlay, description);
}
bool Window::SetThumbarButtons(mate::Arguments* args) {