Use a QTreeWidget instead of QTableWidget.

Even though Qt's widget naming suggests otherwise, a QTreeWidget is
usually a better choice for tables than QTableWidget. The former gives
you a nice, clean Plain Old Table while the latter gives you something
that looks and acts like a spreadsheet.

In this particular instance using QTreeWidget also gives us the option
of adding sub-items with detailed information. Do so for attached
addresses.

Allow sorting by traffic while we're here. Simplify the column hiding
logic. Make sure the sparkline delegate isn't editable.

Change-Id: Ia36ba2e12c1c0cb86ae5b2154e6afcf6549ae049
Reviewed-on: https://code.wireshark.org/review/3466
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Tested-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>