- 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
.SetMethod("isFocused", &Window::IsFocused)
      .SetMethod("show", &Window::Show)
      .SetMethod("showInactive", &Window::ShowInactive)
      .SetMethod("hide", &Window::Hide)
      .SetMethod("isVisible", &Window::IsVisible)
      .SetMethod("maximize", &Window::Maximize)
      .SetMethod("unmaximize", &Window::Unmaximize)
      .SetMethod("isMaximized", &Window::IsMaximized)
      .SetMethod("minimize", &Window::Minimize)
      .SetMethod("restore", &Window::Restore)
      .SetMethod("isMinimized", &Window::IsMinimized)
      .SetMethod("setFullScreen", &Window::SetFullScreen)
      .SetMethod("isFullScreen", &Window::IsFullscreen)
      .SetMethod("setAspectRatio", &Window::SetAspectRatio)
      .SetMethod("getNativeWindowHandle", &Window::GetNativeWindowHandle)
      .SetMethod("getBounds", &Window::GetBounds)
      .SetMethod("setBounds", &Window::SetBounds)
      .SetMethod("getSize", &Window::GetSize)
      .SetMethod("setSize", &Window::SetSize)
      .SetMethod("getContentSize", &Window::GetContentSize)
      .SetMethod("setContentSize", &Window::SetContentSize)
      .SetMethod("setMinimumSize", &Window::SetMinimumSize)
      .SetMethod("getMinimumSize", &Window::GetMinimumSize)
      .SetMethod("setMaximumSize", &Window::SetMaximumSize)
      .SetMethod("getMaximumSize", &Window::GetMaximumSize)
      .SetMethod("setSheetOffset", &Window::SetSheetOffset)
      .SetMethod("setResizable", &Window::SetResizable)
      .SetMethod("isResizable", &Window::IsResizable)
      .SetMethod("setMovable", &Window::SetMovable)
      .SetMethod("isMovable", &Window::IsMovable)