**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
args->ThrowError("Must pass valid certificate object.");
    return;
  }
  std::string data;
  if (!cert_data.Get("data", &data))
    return;
  auto certs = net::X509Certificate::CreateCertificateListFromBytes(
      data.c_str(), data.length(), net::X509Certificate::FORMAT_AUTO);
  if (!certs.empty()) {
    scoped_refptr<net::X509Certificate> cert(certs[0].get());
    for (size_t i = 0; i < identities->size(); ++i) {
      if (cert->EqualsExcludingChain((*identities)[i]->certificate())) {
        net::ClientCertIdentity::SelfOwningAcquirePrivateKey(
            std::move((*identities)[i]),
            base::Bind(&GotPrivateKey, delegate, std::move(cert)));
        break;
      }
    }
  }
}
void PassLoginInformation(scoped_refptr<LoginHandler> login_handler,
                          mate::Arguments* args) {
  base::string16 username, password;
  if (args->GetNext(&username) && args->GetNext(&password))
    login_handler->Login(username, password);
  else
    login_handler->CancelAuth();
}
#if defined(USE_NSS_CERTS)
int ImportIntoCertStore(CertificateManagerModel* model,