**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
const App::FileIconCallback& callback,
                         gfx::Image* icon) {
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  if (icon && !icon->IsEmpty()) {
    callback.Run(v8::Null(isolate), *icon);
  } else {
    v8::Local<v8::String> error_message =
        v8::String::NewFromUtf8(isolate, "Failed to get file icon.");
    callback.Run(v8::Exception::Error(error_message), gfx::Image());
  }
}
}  
App::App(v8::Isolate* isolate) {
  static_cast<AtomBrowserClient*>(AtomBrowserClient::Get())->set_delegate(this);
  Browser::Get()->AddObserver(this);
  content::GpuDataManager::GetInstance()->AddObserver(this);
  base::ProcessId pid = base::GetCurrentProcId();
  auto process_metric = std::make_unique<atom::ProcessMetric>(
      content::PROCESS_TYPE_BROWSER, pid,
      base::ProcessMetrics::CreateCurrentProcessMetrics());
  app_metrics_[pid] = std::move(process_metric);
  Init(isolate);
}
App::~App() {