**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
std::string result_code;
    switch (val) {
      case JumpListResult::SUCCESS:
        result_code = "ok";
        break;
      case JumpListResult::ARGUMENT_ERROR:
        result_code = "argumentError";
        break;
      case JumpListResult::GENERIC_ERROR:
        result_code = "error";
        break;
      case JumpListResult::CUSTOM_CATEGORY_SEPARATOR_ERROR:
        result_code = "invalidSeparatorError";
        break;
      case JumpListResult::MISSING_FILE_TYPE_REGISTRATION_ERROR:
        result_code = "fileTypeRegistrationError";
        break;
      case JumpListResult::CUSTOM_CATEGORY_ACCESS_DENIED_ERROR:
        result_code = "customCategoryAccessDeniedError";
        break;
    }
    return ConvertToV8(isolate, result_code);
  }
};
#endif
template <>
struct Converter<Browser::LoginItemSettings> {
  static bool FromV8(v8::Isolate* isolate,