Add Decode As functionality for MPLS PW Associated Channel Type.

Add CFM and MPLS dissector as possible decoders.

Bug: 10574
Change-Id: Ic39c6b8d68b965ff9d342f5ee789a8eff3134a5a
Reviewed-on: https://code.wireshark.org/review/4935
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>