fix KCategorizedView race

BUG: 213068
BUG: 287847
REVIEW: 103335

QListView::updateGeometries() has it's own opinion on whether the scrollbars should be visible (valid range) or not
and triggers a (sometimes additionally timered) resize through ::layoutChildren()
http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/itemviews/qlistview.cpp#line1499
(the comment above the main block isn't all accurate, layoutChldren is called regardless of the policy)

As a result QListView and KCategorizedView occasionally started a race on the scrollbar visibility, effectively blocking the UI
So we prevent QListView from having an own opinion on the scrollbar visibility by
fixing it before calling the baseclass QListView::updateGeometries() and restoring the policy afterwards