* Change `var` to `let`.
* Change `function() {}` to `() => {}`.
* Use shorthand function syntax on object notation.
* Remove spaces between object notation brackets.
* Small fixes.
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.
There was some confusion about the correct way to add an `All Files` filter (see #2525), so I added it to the example.
Also added a short note about the extension syntax.
This is related to PR #2547.
Added a comment about the fact that on Windows, the "question" message box uses the same icon as the "info" message box (Electron uses the task dialog API instead of the message box API). This is because Microsoft's API doesn't supply a default question icon (see `pszIcon` in https://msdn.microsoft.com/en-us/library/windows/desktop/bb760540(v=vs.85).aspx).
