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
void BrowserChildProcessKilled(const content::ChildProcessData& data,
                                 int exit_code) override;
 private:
  void SetAppPath(const base::FilePath& app_path);
  void ChildProcessLaunched(int process_type, base::ProcessHandle handle);
  void ChildProcessDisconnected(base::ProcessId pid);
  
  base::FilePath GetPath(mate::Arguments* args, const std::string& name);
  void SetPath(mate::Arguments* args,
               const std::string& name,
               const base::FilePath& path);
  void SetDesktopName(const std::string& desktop_name);
  std::string GetLocale();
  bool MakeSingleInstance(
      const ProcessSingleton::NotificationCallback& callback);
  void ReleaseSingleInstance();
  bool Relaunch(mate::Arguments* args);
  void DisableHardwareAcceleration(mate::Arguments* args);
  void DisableDomainBlockingFor3DAPIs(mate::Arguments* args);
  bool IsAccessibilitySupportEnabled();
  void SetAccessibilitySupportEnabled(bool enabled);
  Browser::LoginItemSettings GetLoginItemSettings(mate::Arguments* args);
#if defined(USE_NSS_CERTS)
  void ImportCertificate(const base::DictionaryValue& options,
                         const net::CompletionCallback& callback);
#endif
  void GetFileIcon(const base::FilePath& path, mate::Arguments* args);
  std::vector<mate::Dictionary> GetAppMetrics(v8::Isolate* isolate);
  v8::Local<v8::Value> GetGPUFeatureStatus(v8::Isolate* isolate);
  void EnableMixedSandbox(mate::Arguments* args);