Qt: Cache packet list column strings.

For each displayed packet list row, save a copy of or a pointer to
column strings similar to ui/gtk/packet_list_store.c. This lets us call
epan_dissect_run only once per row.

Bug: 9511
Change-Id: I17e8ebeb5ed70518c9047413c3b2a46f01e904ef
Reviewed-on: https://code.wireshark.org/review/2752
Reviewed-by: Anders Broman <a.broman58@gmail.com>