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
return ui::cocoa::AtomBundleMover::Move(args);
}
bool App::IsInApplicationsFolder() {
  return ui::cocoa::AtomBundleMover::IsCurrentAppInApplicationsFolder();
}
#endif
mate::Handle<App> App::Create(v8::Isolate* isolate) {
  return mate::CreateHandle(isolate, new App(isolate));
}
void App::BuildPrototype(v8::Isolate* isolate,
                         v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "App"));
  auto browser = base::Unretained(Browser::Get());
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .SetMethod("quit", base::Bind(&Browser::Quit, browser))
      .SetMethod("exit", base::Bind(&Browser::Exit, browser))
      .SetMethod("focus", base::Bind(&Browser::Focus, browser))
      .SetMethod("getVersion", base::Bind(&Browser::GetVersion, browser))
      .SetMethod("setVersion", base::Bind(&Browser::SetVersion, browser))
      .SetMethod("getName", base::Bind(&Browser::GetName, browser))
      .SetMethod("setName", base::Bind(&Browser::SetName, browser))
      .SetMethod("isReady", base::Bind(&Browser::is_ready, browser))
      .SetMethod("addRecentDocument",
                 base::Bind(&Browser::AddRecentDocument, browser))
      .SetMethod("clearRecentDocuments",
                 base::Bind(&Browser::ClearRecentDocuments, browser))