**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,
    net::SSLCertRequestInfo* cert_request_info,
    net::ClientCertIdentityList identities,
    std::unique_ptr<content::ClientCertificateDelegate> delegate) {
  std::shared_ptr<content::ClientCertificateDelegate> shared_delegate(
      delegate.release());
  
  
  auto client_certs = net::CertificateList();
  for (const std::unique_ptr<net::ClientCertIdentity>& identity : identities)
    client_certs.push_back(identity->certificate());
  auto shared_identities =
      std::make_shared<net::ClientCertIdentityList>(std::move(identities));
  bool prevent_default =
      Emit("select-client-certificate",
           WebContents::FromOrCreate(isolate(), web_contents),
           cert_request_info->host_and_port.ToString(), std::move(client_certs),
           base::Bind(&OnClientCertificateSelected, isolate(), shared_delegate,
                      shared_identities));
  
  if (!prevent_default) {
    scoped_refptr<net::X509Certificate> cert =
        (*shared_identities)[0]->certificate();
    net::ClientCertIdentity::SelfOwningAcquirePrivateKey(
        std::move((*shared_identities)[0]),
        base::Bind(&GotPrivateKey, shared_delegate, std::move(cert)));
  }
}
void App::OnGpuProcessCrashed(base::TerminationStatus status) {