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.h
#ifndef ATOM_BROWSER_API_ATOM_API_APP_H_
#define ATOM_BROWSER_API_ATOM_API_APP_H_
#include <memory>
#include <string>
#include <utility>
#include <vector>
#include "atom/browser/api/event_emitter.h"
#include "atom/browser/atom_browser_client.h"
#include "atom/browser/browser.h"
#include "atom/browser/browser_observer.h"
#include "atom/common/native_mate_converters/callback.h"
#include "base/process/process_iterator.h"
#include "base/task/cancelable_task_tracker.h"
#include "chrome/browser/icon_manager.h"
#include "chrome/browser/process_singleton.h"
#include "content/public/browser/browser_child_process_observer.h"
#include "content/public/browser/gpu_data_manager_observer.h"
#include "content/public/browser/render_process_host.h"
#include "native_mate/dictionary.h"
#include "native_mate/handle.h"
#include "net/base/completion_callback.h"
#include "net/ssl/client_cert_identity.h"
#if defined(USE_NSS_CERTS)
#include "chrome/browser/certificate_manager_model.h"
#endif
namespace base {
class FilePath;