Add a "placeholder protocol" for H.223 to avoid confusion when using Decode As.

The Decode As dialog presents two H.223 options and only "trial and error" can determine if you want bitswapping or not.

Change-Id: I9303c73375d130d3097d347bb374019dfc714068
Reviewed-on: https://code.wireshark.org/review/4909
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>