Refactor "common" Conversation table functionality.

Refactor (non-GUI) conversation table functionality from gtk/Qt to epan.  Also refactor "common GUI" conversation table functionality.

The idea is to not have to modify the GUI when a dissector adds a new "conversation type"

Change-Id: I11f08d0d7edd631218663ba4b902c4a4c849acda
Reviewed-on: https://code.wireshark.org/review/3113
Reviewed-by: Gerald Combs <gerald@wireshark.org>