Fix TableView scrolling problem

TableView was specifying its content width based on item rows' width.
Item rows get dynamically loaded and destroyed while scrolling, and it
sometimes applied a width of a half-(de)constructed item row. That made
the horizontal scrollbar jump to the beginning. This patch makes
TableView use header row's (static) width for specifying the content
width.

Task-number: QTBUG-34344
Change-Id: I797f0f5d294fbb8c95b2836f9aec0dc14c32bc6b
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>