**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
scoped_refptr<net::X509Certificate> cert,
                   scoped_refptr<net::SSLPrivateKey> private_key) {
  delegate->ContinueWithCertificate(cert, private_key);
}
void OnClientCertificateSelected(
    v8::Isolate* isolate,
    std::shared_ptr<content::ClientCertificateDelegate> delegate,
    std::shared_ptr<net::ClientCertIdentityList> identities,
    mate::Arguments* args) {
  if (args->Length() == 2) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }
  v8::Local<v8::Value> val;
  args->GetNext(&val);
  if (val->IsNull()) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }
  mate::Dictionary cert_data;
  if (!mate::ConvertFromV8(isolate, val, &cert_data)) {