**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
JumpListCategory::Type val) {
    std::string category_type;
    switch (val) {
      case JumpListCategory::Type::TASKS:
        category_type = "tasks";
        break;
      case JumpListCategory::Type::FREQUENT:
        category_type = "frequent";
        break;
      case JumpListCategory::Type::RECENT:
        category_type = "recent";
        break;
      case JumpListCategory::Type::CUSTOM:
        category_type = "custom";
        break;
    }
    return mate::ConvertToV8(isolate, category_type);
  }
};
template <>
struct Converter<JumpListCategory> {
  static bool FromV8(v8::Isolate* isolate,