**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
v8::Local<v8::Value> val,
                     JumpListItem* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    if (!dict.Get("type", &(out->type)))
      return false;
    switch (out->type) {
      case JumpListItem::Type::TASK:
        if (!dict.Get("program", &(out->path)) ||
            !dict.Get("title", &(out->title)))
          return false;
        if (dict.Get("iconPath", &(out->icon_path)) &&
            !dict.Get("iconIndex", &(out->icon_index)))
          return false;
        dict.Get("args", &(out->arguments));
        dict.Get("description", &(out->description));
        return true;
      case JumpListItem::Type::SEPARATOR:
        return true;
      case JumpListItem::Type::FILE:
        return dict.Get("path", &(out->path));
    }
    assert(false);
    return false;
  }
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,