**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
auto* ax_state = content::BrowserAccessibilityState::GetInstance();
  if (enabled) {
    ax_state->OnScreenReaderDetected();
  } else {
    ax_state->DisableAccessibility();
  }
  Browser::Get()->OnAccessibilitySupportChanged();
}
Browser::LoginItemSettings App::GetLoginItemSettings(mate::Arguments* args) {
  Browser::LoginItemSettings options;
  args->GetNext(&options);
  return Browser::Get()->GetLoginItemSettings(options);
}
#if defined(USE_NSS_CERTS)
void App::ImportCertificate(const base::DictionaryValue& options,
                            const net::CompletionCallback& callback) {
  auto browser_context = AtomBrowserContext::From("", false);
  if (!certificate_manager_model_) {
    auto copy = base::DictionaryValue::From(
        base::Value::ToUniquePtrValue(options.Clone()));
    CertificateManagerModel::Create(
        browser_context.get(),
        base::Bind(&App::OnCertificateManagerModelCreated,
                   base::Unretained(this), base::Passed(&copy), callback));
    return;
  }
  int rv = ImportIntoCertStore(certificate_manager_model_.get(), options);
  callback.Run(rv);
}
void App::OnCertificateManagerModelCreated(