**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::Type* out) {
    std::string category_type;
    if (!ConvertFromV8(isolate, val, &category_type))
      return false;
    if (category_type == "tasks")
      *out = JumpListCategory::Type::TASKS;
    else if (category_type == "frequent")
      *out = JumpListCategory::Type::FREQUENT;
    else if (category_type == "recent")
      *out = JumpListCategory::Type::RECENT;
    else if (category_type == "custom")
      *out = JumpListCategory::Type::CUSTOM;
    else
      return false;
    return true;
  }
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,