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
.SetMethod("setJumpList", &App::SetJumpList)
#endif
#if defined(OS_LINUX)
      .SetMethod("isUnityRunning",
                 base::Bind(&Browser::IsUnityRunning, browser))
#endif
      .SetMethod("setAppPath", &App::SetAppPath)
      .SetMethod("getAppPath", &App::GetAppPath)
      .SetMethod("setPath", &App::SetPath)
      .SetMethod("getPath", &App::GetPath)
      .SetMethod("setDesktopName", &App::SetDesktopName)
      .SetMethod("getLocale", &App::GetLocale)
#if defined(USE_NSS_CERTS)
      .SetMethod("importCertificate", &App::ImportCertificate)
#endif
      .SetMethod("makeSingleInstance", &App::MakeSingleInstance)
      .SetMethod("releaseSingleInstance", &App::ReleaseSingleInstance)
      .SetMethod("relaunch", &App::Relaunch)
      .SetMethod("isAccessibilitySupportEnabled",
                 &App::IsAccessibilitySupportEnabled)
      .SetMethod("setAccessibilitySupportEnabled",
                 &App::SetAccessibilitySupportEnabled)
      .SetMethod("disableHardwareAcceleration",
                 &App::DisableHardwareAcceleration)
      .SetMethod("disableDomainBlockingFor3DAPIs",
                 &App::DisableDomainBlockingFor3DAPIs)
      .SetMethod("getFileIcon", &App::GetFileIcon)
      .SetMethod("getAppMetrics", &App::GetAppMetrics)
      .SetMethod("getGPUFeatureStatus", &App::GetGPUFeatureStatus)
#if defined(OS_MACOSX)