**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
if (Emit("open-file", file_path)) {
    *prevent_default = true;
  }
}
void App::OnOpenURL(const std::string& url) {
  Emit("open-url", url);
}
void App::OnActivate(bool has_visible_windows) {
  Emit("activate", has_visible_windows);
}
void App::OnWillFinishLaunching() {
  Emit("will-finish-launching");
}
void App::OnFinishLaunching(const base::DictionaryValue& launch_info) {
#if defined(OS_LINUX)
  
  
  media::AudioManager::SetGlobalAppName(Browser::Get()->GetName());
#endif
  Emit("ready", launch_info);
}
void App::OnPreMainMessageLoopRun() {
  content::BrowserChildProcessObserver::Add(this);
  if (process_singleton_) {
    process_singleton_->OnBrowserReady();
  }
}
void App::OnAccessibilitySupportChanged() {