Qt: Always make the packet list row heights uniform.

Qt: Always make the packet list row heights uniform.

In tests here using GTK+ 2.24 and 3.10, GtkTreeView handles multi-line
items by adjusting the height for all rows, but only after the number of
multi-line items exceeds some sort of threshold. For a packet capture
which contains a few DNS packets and a lot of TCP packets, if I change
"Standard query" to "Standard\nquery" in packet-dns.c I get
single-height packet list items. If I change "[TCP segment of a
reassembled PDU]" to "[TCP segment of a\nreassembled PDU]" in
packet-tcp.c (which results in more multi-line column strings) I get
double-height packet list items.

The current Qt code initially sets the uniformRowHeights property then
falls back to variable row heights if we run across a multi-line column
string. This adds a lot of logic which can impact other functionality
(e.g. column widths) and recalculating row heights is painfully slow for
large numbers of packets.

Instead of trying to manage variable row heights, always enable
uniformRowHeights. Track the maximum newline count and trigger a row
height adjustment when it changes. This mimics the GTK+ UI behavior,
although it should be more reliable.

Note that we need to adjust some numbers in RelatedPacketDelegate.

Change-Id: I289e963b6f00338c4374e602fa3fc83d04554519
Ping-Bug: 11515
Ping-Bug: 10924
Reviewed-on: https://code.wireshark.org/review/10628
Reviewed-by: Gerald Combs <gerald@wireshark.org>