**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
void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  auto* command_line = base::CommandLine::ForCurrentProcess();
  mate::Dictionary dict(isolate, exports);
  dict.Set("App", atom::api::App::GetConstructor(isolate)->GetFunction());
  dict.Set("app", atom::api::App::Create(isolate));
  dict.SetMethod("appendSwitch", &AppendSwitch);
  dict.SetMethod("appendArgument", base::Bind(&base::CommandLine::AppendArg,
                                              base::Unretained(command_line)));
#if defined(OS_MACOSX)
  auto browser = base::Unretained(Browser::Get());
  dict.SetMethod("dockBounce", &DockBounce);
  dict.SetMethod("dockCancelBounce",
                 base::Bind(&Browser::DockCancelBounce, browser));
  dict.SetMethod("dockDownloadFinished",
                 base::Bind(&Browser::DockDownloadFinished, browser));
  dict.SetMethod("dockSetBadgeText",
                 base::Bind(&Browser::DockSetBadgeText, browser));
  dict.SetMethod("dockGetBadgeText",
                 base::Bind(&Browser::DockGetBadgeText, browser));
  dict.SetMethod("dockHide", base::Bind(&Browser::DockHide, browser));
  dict.SetMethod("dockShow", base::Bind(&Browser::DockShow, browser));
  dict.SetMethod("dockIsVisible", base::Bind(&Browser::DockIsVisible, browser));
  dict.SetMethod("dockSetMenu", &DockSetMenu);
  dict.SetMethod("dockSetIcon", base::Bind(&Browser::DockSetIcon, browser));
#endif
}
}  
NODE_BUILTIN_MODULE_CONTEXT_AWARE(atom_browser_app, Initialize)