Have ConversationDialog retain data between captures.

Remove our tap listeners when the capture file closes. This lets us view
conversation data for multiple capture files.
Change-Id: Ifaa1d2c9d3cd10b1d282aa0070ee2edacc7ce3b0
Reviewed-on: https://code.wireshark.org/review/3376
Reviewed-by: Gerald Combs <gerald@wireshark.org>