- 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
return sheet_offset_;
}
void NativeWindow::SetRepresentedFilename(const std::string& filename) {
}
std::string NativeWindow::GetRepresentedFilename() {
  return "";
}
void NativeWindow::SetDocumentEdited(bool edited) {
}
bool NativeWindow::IsDocumentEdited() {
  return false;
}
void NativeWindow::SetIgnoreMouseEvents(bool ignore) {
}
void NativeWindow::SetMenu(ui::MenuModel* menu) {
}
bool NativeWindow::HasModalDialog() {
  return has_dialog_attached_;
}
void NativeWindow::FocusOnWebView() {
  web_contents()->GetRenderViewHost()->GetWidget()->Focus();
}
void NativeWindow::BlurWebView() {
  web_contents()->GetRenderViewHost()->GetWidget()->Blur();
}
bool NativeWindow::IsWebViewFocused() {
  auto host_view = web_contents()->GetRenderViewHost()->GetWidget()->GetView();
  return host_view && host_view->HasFocus();
}
void NativeWindow::CapturePage(const gfx::Rect& rect,