**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
const base::DictionaryValue& options) {
  std::string file_data, cert_path;
  base::string16 password;
  net::ScopedCERTCertificateList imported_certs;
  int rv = -1;
  options.GetString("certificate", &cert_path);
  options.GetString("password", &password);
  if (!cert_path.empty()) {
    if (base::ReadFileToString(base::FilePath(cert_path), &file_data)) {
      auto module = model->cert_db()->GetPrivateSlot();
      rv = model->ImportFromPKCS12(module.get(), file_data, password, true,
                                   &imported_certs);
      if (imported_certs.size() > 1) {
        auto it = imported_certs.begin();
        ++it;  
        for (; it != imported_certs.end(); ++it)
          rv &= model->SetCertTrust(it->get(), net::CA_CERT,
                                    net::NSSCertDatabase::TRUSTED_SSL);
      }
    }
  }
  return rv;
}
#endif
void OnIconDataAvailable(v8::Isolate* isolate,