- 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
Emit("moved");
}
void Window::OnWindowEnterFullScreen() {
  Emit("enter-full-screen");
}
void Window::OnWindowLeaveFullScreen() {
  Emit("leave-full-screen");
}
void Window::OnWindowScrollTouchBegin() {
  Emit("scroll-touch-begin");
}
void Window::OnWindowScrollTouchEnd() {
  Emit("scroll-touch-end");
}
void Window::OnWindowSwipe(const std::string& direction) {
  Emit("swipe", direction);
}
void Window::OnWindowEnterHtmlFullScreen() {
  Emit("enter-html-full-screen");
}
void Window::OnWindowLeaveHtmlFullScreen() {
  Emit("leave-html-full-screen");
}
void Window::OnRendererUnresponsive() {
  Emit("unresponsive");
}
void Window::OnRendererResponsive() {
  Emit("responsive");
}
void Window::OnExecuteWindowsCommand(const std::string& command_name) {