Add ToadletContext-dependent showAsToadlet()

- This is useful for plugins which need to decide which Toadlet to
  return using the session cookies which can only be obtained through
  the ToadletContext.
- The only caller of showAsToadlet() without arguments had a
  ToadletContext available.
- The method taking ToadletContext calls that which does not by to
  support existing uses and overrides.