**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 "atom/browser/api/atom_api_app.h"
#include <string>
#include <vector>
#include "atom/browser/api/atom_api_menu.h"
#include "atom/browser/api/atom_api_session.h"
#include "atom/browser/api/atom_api_web_contents.h"
#include "atom/browser/api/gpuinfo_manager.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/atom_paths.h"
#include "atom/browser/login_handler.h"
#include "atom/browser/relauncher.h"
#include "atom/common/atom_command_line.h"
#include "atom/common/native_mate_converters/callback.h"
#include "atom/common/native_mate_converters/file_path_converter.h"
#include "atom/common/native_mate_converters/gurl_converter.h"
#include "atom/common/native_mate_converters/image_converter.h"
#include "atom/common/native_mate_converters/net_converter.h"
#include "atom/common/native_mate_converters/network_converter.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "base/command_line.h"
#include "base/environment.h"
#include "base/files/file_path.h"