**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
content::WebContents* web_contents,
    int cert_error,
    const net::SSLInfo& ssl_info,
    const GURL& request_url,
    content::ResourceType resource_type,
    bool strict_enforcement,
    bool expired_previous_decision,
    const base::Callback<void(content::CertificateRequestResultType)>&
        callback) {
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default = Emit(
      "certificate-error", WebContents::FromOrCreate(isolate(), web_contents),
      request_url, net::ErrorToString(cert_error), ssl_info.cert, callback);
  
  if (!prevent_default)
    callback.Run(content::CERTIFICATE_REQUEST_RESULT_TYPE_DENY);
}
void App::SelectClientCertificate(