**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,
                     JumpListCategory* out) {
    mate::Dictionary dict;
    if (!ConvertFromV8(isolate, val, &dict))
      return false;
    if (dict.Get("name", &(out->name)) && out->name.empty())
      return false;
    if (!dict.Get("type", &(out->type))) {
      if (out->name.empty())
        out->type = JumpListCategory::Type::TASKS;
      else
        out->type = JumpListCategory::Type::CUSTOM;
    }
    if ((out->type == JumpListCategory::Type::TASKS) ||
        (out->type == JumpListCategory::Type::CUSTOM)) {
      if (!dict.Get("items", &(out->items)))
        return false;
    }
    return true;
  }
};
template <>
struct Converter<JumpListResult> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate, JumpListResult val) {