**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
ChildProcessLaunched(content::PROCESS_TYPE_RENDERER,
                       host->GetProcess().Handle());
}
void App::RenderProcessDisconnected(base::ProcessId host_pid) {
  ChildProcessDisconnected(host_pid);
}
void App::ChildProcessLaunched(int process_type, base::ProcessHandle handle) {
  auto pid = base::GetProcId(handle);
#if defined(OS_MACOSX)
  std::unique_ptr<base::ProcessMetrics> metrics(
      base::ProcessMetrics::CreateProcessMetrics(
          handle, content::BrowserChildProcessHost::GetPortProvider()));
#else
  std::unique_ptr<base::ProcessMetrics> metrics(
      base::ProcessMetrics::CreateProcessMetrics(handle));
#endif
  app_metrics_[pid] = std::make_unique<atom::ProcessMetric>(process_type, pid,
                                                            std::move(metrics));
}
void App::ChildProcessDisconnected(base::ProcessId pid) {
  app_metrics_.erase(pid);
}
base::FilePath App::GetAppPath() const {
  return app_path_;
}
void App::SetAppPath(const base::FilePath& app_path) {