Currently, when a user calls `makeSingleInstance` they need to handle the second instance in a callback.

A better pattern for both users and third-party modules who want to use this information would be to emit an event on the `app` module when the second instance starts.

A pattern like:

```js
app.makeSingleInstance();

app.on('second-instance', (e, argv) => {
  // do-stuff
});
```

As step 1 I'll introduce support for the event and deprecate the callback.  Then in a major version, we can remove the callback.
atom/browser/api/atom_api_app.cc
JumpListItem::Type val) {
    std::string item_type;
    switch (val) {
      case JumpListItem::Type::TASK:
        item_type = "task";
        break;
      case JumpListItem::Type::SEPARATOR:
        item_type = "separator";
        break;
      case JumpListItem::Type::FILE:
        item_type = "file";
        break;
    }
    return mate::ConvertToV8(isolate, item_type);
  }
};
template <>
struct Converter<JumpListItem> {
  static bool FromV8(v8::Isolate* isolate,