VoIP calls: Start separating common and GTK+-dependent code.

Remove more calls to voip_calls_get_info. Get rid of the global cfile.
Lowercase more function names.

Change-Id: Ieda779779cce9f05e07c54314bceab4a18bae22f
Reviewed-on: https://code.wireshark.org/review/5324
Reviewed-by: Gerald Combs <gerald@wireshark.org>