- 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
std::vector<int> result(2);
  gfx::Size size = window_->GetMinimumSize();
  result[0] = size.width();
  result[1] = size.height();
  return result;
}
void Window::SetMaximumSize(int width, int height) {
  window_->SetMaximumSize(gfx::Size(width, height));
}
std::vector<int> Window::GetMaximumSize() {
  std::vector<int> result(2);
  gfx::Size size = window_->GetMaximumSize();
  result[0] = size.width();
  result[1] = size.height();
  return result;
}
void Window::SetSheetOffset(double offset) {
  window_->SetSheetOffset(offset);
}
void Window::SetResizable(bool resizable) {
  window_->SetResizable(resizable);
}
bool Window::IsResizable() {
  return window_->IsResizable();
}
void Window::SetMovable(bool movable) {
  window_->SetMovable(movable);
}
bool Window::IsMovable() {