**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
.SetMethod("isUnityRunning",
                 base::Bind(&Browser::IsUnityRunning, browser))
#endif
      .SetMethod("setAppPath", &App::SetAppPath)
      .SetMethod("getAppPath", &App::GetAppPath)
      .SetMethod("setPath", &App::SetPath)
      .SetMethod("getPath", &App::GetPath)
      .SetMethod("setDesktopName", &App::SetDesktopName)
      .SetMethod("getLocale", &App::GetLocale)
#if defined(USE_NSS_CERTS)
      .SetMethod("importCertificate", &App::ImportCertificate)
#endif
      .SetMethod("hasSingleInstanceLock", &App::HasSingleInstanceLock)
      .SetMethod("requestSingleInstanceLock", &App::RequestSingleInstanceLock)
      .SetMethod("releaseSingleInstanceLock", &App::ReleaseSingleInstanceLock)
      .SetMethod("relaunch", &App::Relaunch)
      .SetMethod("isAccessibilitySupportEnabled",
                 &App::IsAccessibilitySupportEnabled)
      .SetMethod("setAccessibilitySupportEnabled",
                 &App::SetAccessibilitySupportEnabled)
      .SetMethod("disableHardwareAcceleration",
                 &App::DisableHardwareAcceleration)
      .SetMethod("disableDomainBlockingFor3DAPIs",
                 &App::DisableDomainBlockingFor3DAPIs)
      .SetMethod("getFileIcon", &App::GetFileIcon)
      .SetMethod("getAppMetrics", &App::GetAppMetrics)
      .SetMethod("getGPUFeatureStatus", &App::GetGPUFeatureStatus)
      .SetMethod("getGPUInfo", &App::GetGPUInfo)
#if defined(OS_MACOSX)
      .SetMethod("moveToApplicationsFolder", &App::MoveToApplicationsFolder)