Adds a new "setSheetOffset" API to the `dialog` module, which allows you to change the attachment point for sheets on Mac OS X. I put the API on the dialog module, even though Mac OS X requires that the native window hold and return the desired offset.
1. I was originally hoping to make this an argument on the actual dialog.show* calls, but it seems the parameter set is defined in `libchromiumcontent` and I wasn't sure it would be appropriate to add there?
2. The API could also be on the BrowserWindow (eg `BrowserWindow.setSheetOffset`). I don't have a strong preference, but I think it's more discoverable on the `dialog` module.
