QHeaderView::ResizeToContents slow hide fix

QHeaderView::ResizeToContents slow hide fix

I have been a bit confused about what QHeaderView::ResizeToContents
does good. It only sizes depending of the visible part.

However in hide it goes crazy and checks calculates size hints
for every row. At first I considered to solve it in QTableView.
It could be made to calculate the maximum height of the maximum
screen - however I decided not to touch it since I was not
certain that it wouldn't have any unexpected side effects.

Therefore I instead made this patch which is more simple and seems
to be safer. The logic is that when a QHeaderView is hidden we actually
only want to recalculate sizes if the headerview has a visible parent.

Task-number: QTBUG-14234

Change-Id: I186ab6afa95aed43022f2bb7c36e3fd008355d9b
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>