**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
v8::Local<v8::Value> val,
                     content::CertificateRequestResultType* out) {
    bool b;
    if (!ConvertFromV8(isolate, val, &b))
      return false;
    *out = b ? content::CERTIFICATE_REQUEST_RESULT_TYPE_CONTINUE
             : content::CERTIFICATE_REQUEST_RESULT_TYPE_CANCEL;
    return true;
  }
};
}  
namespace atom {
ProcessMetric::ProcessMetric(int type,
                             base::ProcessId pid,
                             std::unique_ptr<base::ProcessMetrics> metrics) {
  this->type = type;
  this->pid = pid;
  this->metrics = std::move(metrics);
}
ProcessMetric::~ProcessMetric() = default;
namespace api {
namespace {
class AppIdProcessIterator : public base::ProcessIterator {