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
#include "chrome/browser/icon_manager.h"
#include "chrome/common/chrome_paths.h"
#include "content/browser/gpu/compositor_util.h"
#include "content/browser/gpu/gpu_data_manager_impl.h"
#include "content/public/browser/browser_accessibility_state.h"
#include "content/public/browser/browser_child_process_host.h"
#include "content/public/browser/child_process_data.h"
#include "content/public/browser/client_certificate_delegate.h"
#include "content/public/browser/gpu_data_manager.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/common/content_switches.h"
#include "media/audio/audio_manager.h"
#include "native_mate/object_template_builder.h"
#include "net/ssl/client_cert_identity.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "ui/base/l10n/l10n_util.h"
#include "ui/gfx/image/image.h"
#if defined(OS_WIN)
#include "atom/browser/ui/win/jump_list.h"
#include "base/strings/utf_string_conversions.h"
#endif
#if defined(OS_MACOSX)
#include "atom/browser/ui/cocoa/atom_bundle_mover.h"
#endif
using atom::Browser;
namespace mate {
#if defined(OS_WIN)
template <>
struct Converter<Browser::UserTask> {
  static bool FromV8(v8::Isolate* isolate,