**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
static_cast<AtomBrowserClient*>(AtomBrowserClient::Get())
      ->set_delegate(nullptr);
  Browser::Get()->RemoveObserver(this);
  content::GpuDataManager::GetInstance()->RemoveObserver(this);
  content::BrowserChildProcessObserver::Remove(this);
}
void App::OnBeforeQuit(bool* prevent_default) {
  if (Emit("before-quit")) {
    *prevent_default = true;
  }
}
void App::OnWillQuit(bool* prevent_default) {
  if (Emit("will-quit")) {
    *prevent_default = true;
  }
}
void App::OnWindowAllClosed() {
  Emit("window-all-closed");
}
void App::OnQuit() {
  int exitCode = AtomBrowserMainParts::Get()->GetExitCode();
  Emit("quit", exitCode);
  if (process_singleton_) {
    process_singleton_->Cleanup();
    process_singleton_.reset();
  }
}
void App::OnOpenFile(bool* prevent_default, const std::string& file_path) {