**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
#include "base/files/file_util.h"
#include "base/path_service.h"
#include "base/strings/string_util.h"
#include "base/sys_info.h"
#include "chrome/browser/browser_process.h"
#include "chrome/browser/icon_manager.h"
#include "chrome/common/chrome_paths.h"
#include "content/browser/gpu/compositor_util.h"
#include "content/browser/gpu/gpu_data_manager_impl.h"
#include "content/public/browser/browser_accessibility_state.h"
#include "content/public/browser/browser_child_process_host.h"
#include "content/public/browser/child_process_data.h"
#include "content/public/browser/client_certificate_delegate.h"
#include "content/public/browser/gpu_data_manager.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/common/content_switches.h"
#include "media/audio/audio_manager.h"
#include "native_mate/object_template_builder.h"
#include "net/ssl/client_cert_identity.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "services/network/public/cpp/network_switches.h"
#include "services/service_manager/sandbox/switches.h"
#include "ui/base/l10n/l10n_util.h"
#include "ui/gfx/image/image.h"
#include "atom/common/node_includes.h"  
#if defined(OS_WIN)
#include "atom/browser/ui/win/jump_list.h"
#include "base/strings/utf_string_conversions.h"
#endif
#if defined(OS_MACOSX)