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
public:
  AppIdProcessIterator() : base::ProcessIterator(nullptr) {}
 protected:
  bool IncludeEntry() override {
    return (entry().parent_pid() == base::GetCurrentProcId() ||
            entry().pid() == base::GetCurrentProcId());
  }
};
IconLoader::IconSize GetIconSizeByString(const std::string& size) {