Qt: Packet list column sorting.

Sorting behavior should be identical to GTK+.

Pass the correct position to beginInsertRows. Add a list of to-do items
to packet_list.cpp.

Change-Id: Ie6ab4b9f2d780a2af430d0f90529edca5485dada
Reviewed-on: https://code.wireshark.org/review/4481
Reviewed-by: Gerald Combs <gerald@wireshark.org>