**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
if (name == "appData")
    return DIR_APP_DATA;
  else if (name == "userData")
    return DIR_USER_DATA;
  else if (name == "cache")
    return DIR_CACHE;
  else if (name == "userCache")
    return DIR_USER_CACHE;
  else if (name == "logs")
    return DIR_APP_LOGS;
  else if (name == "home")
    return base::DIR_HOME;
  else if (name == "temp")
    return base::DIR_TEMP;
  else if (name == "userDesktop" || name == "desktop")
    return base::DIR_USER_DESKTOP;
  else if (name == "exe")
    return base::FILE_EXE;
  else if (name == "module")
    return base::FILE_MODULE;
  else if (name == "documents")
    return chrome::DIR_USER_DOCUMENTS;
  else if (name == "downloads")
    return chrome::DIR_DEFAULT_DOWNLOADS;