Refactor "color" conversation filters

Have dissectors register their desire to be part of "color" conversation filters and have the GUI use that registered list.  GUI actually using API will come in a separate commit.

Change-Id: I5ffe922d97894fe7bf3182056b76ab5839a9461a
Reviewed-on: https://code.wireshark.org/review/5658
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>