**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("isInApplicationsFolder", &App::IsInApplicationsFolder)
#endif
#if defined(MAS_BUILD)
      .SetMethod("startAccessingSecurityScopedResource",
                 &App::StartAccessingSecurityScopedResource)
#endif
      .SetMethod("enableSandbox", &App::EnableSandbox)
      .SetMethod("enableMixedSandbox", &App::EnableMixedSandbox);
}
}  
}  
namespace {
void AppendSwitch(const std::string& switch_string, mate::Arguments* args) {
  auto* command_line = base::CommandLine::ForCurrentProcess();
  if (base::EndsWith(switch_string, "-path",
                     base::CompareCase::INSENSITIVE_ASCII) ||
      switch_string == network::switches::kLogNetLog) {
    base::FilePath path;
    args->GetNext(&path);
    command_line->AppendSwitchPath(switch_string, path);
    return;
  }
  std::string value;
  if (args->GetNext(&value))
    command_line->AppendSwitchASCII(switch_string, value);
  else
    command_line->AppendSwitch(switch_string);
}
#if defined(OS_MACOSX)
int DockBounce(const std::string& type) {