Qt: Refactor ConversationDialog for endpoints.

Create a TrafficTableDialog (for lack of a better name) parent class
from the general parts of ConversationDialog. Use it to create
EndpointsDialog.

Move the contents of conversation_tree_widget.{cpp,h} to
conversation_dialog.{cpp,h} to match endpoint_dialog and
traffic_table_dialog.

Fill in GeoIP columns dynamically instead of using a hard-coded limit.

Use "endp_" and "ENDP_" prefixes for a lot of endpoint variables and
defines.

Try to make geoip_db_lookup_ipv4 and geoip_db_lookup_ipv6 more robust.

Clean up some includes. Fix a shadowed variable.

Change-Id: I23054816ac7f8c6edb3b1f01c8536db37ba4122d
Reviewed-on: https://code.wireshark.org/review/3462
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>