**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
int request_id = -1;
  if (type == "critical")
    request_id = Browser::Get()->DockBounce(Browser::BOUNCE_CRITICAL);
  else if (type == "informational")
    request_id = Browser::Get()->DockBounce(Browser::BOUNCE_INFORMATIONAL);
  return request_id;
}
void DockSetMenu(atom::api::Menu* menu) {
  Browser::Get()->DockSetMenu(menu->model());
}
#endif
void Initialize(v8::Local<v8::Object> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,