Qt: Make the overlay scroll bar work with Qt 4.

Qt: Make the overlay scroll bar work with Qt 4.

In Qt 4, QScrollBar::setRange isn't a slot. Add a setChildRange
slot and connect to it instead. Add a QT_VERSION_CHECK breadcrumb in
a comment.

Change-Id: Ie28bbb6bd8249c31154a2fe236667adf1d53df61
Reviewed-on: https://code.wireshark.org/review/15215
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Reviewed-by: Gerald Combs <gerald@wireshark.org>