voip_calls: Move to ui.

Move voip_calls.[ch] to ui. Add callbacks to voip_calls_tapinfo_t.
Remove unused function definitions.

Change-Id: Ib12db7053d53afa81ef2a66dc0cfe681bc624dd2
Reviewed-on: https://code.wireshark.org/review/5379
Reviewed-by: Gerald Combs <gerald@wireshark.org>