**Is your feature request related to a problem? Please describe.**

Currently, the only way to show the About panel on macOS is by creating a `{role: 'about'}` menu item and click it. I need to be able to show it manually.

**Describe the solution you'd like**

Electron already has the method, so it's just about exposing this to JS and documenting it:

https://github.com/electron/electron/blob/8ba271efba691357ab1970c1a553b4e019125574/atom/browser/browser_mac.mm#L376

Would also be useful to be able to pass in the panel options directly to the method to override the defaults.

I would call the method: `app.showAboutPanel(options)`

**Describe alternatives you've considered**

None.
atom/browser/api/atom_api_app.cc
if (HasSingleInstanceLock())
    return true;
  base::FilePath user_dir;
  base::PathService::Get(DIR_USER_DATA, &user_dir);
  auto cb = base::Bind(&App::OnSecondInstance, base::Unretained(this));
  process_singleton_.reset(new ProcessSingleton(
      user_dir, base::Bind(NotificationCallbackWrapper, cb)));
  switch (process_singleton_->NotifyOtherProcessOrCreate()) {
    case ProcessSingleton::NotifyResult::LOCK_ERROR:
    case ProcessSingleton::NotifyResult::PROFILE_IN_USE:
    case ProcessSingleton::NotifyResult::PROCESS_NOTIFIED: {
      process_singleton_.reset();
      return false;
    }
    case ProcessSingleton::NotifyResult::PROCESS_NONE:
    default:  
      return true;
  }
}
void App::ReleaseSingleInstanceLock() {
  if (process_singleton_) {
    process_singleton_->Cleanup();
    process_singleton_.reset();
  }
}
bool App::Relaunch(mate::Arguments* js_args) {